The state is a social apparatus that appears with the division into classes in order to keep society constrained within the prevailing social relations, preventing it from collapsing as a result of the class struggle. It is therefore fundamentally conservative and will present itself as "neutral", above the classes in conflict, although it cannot be anything other than a tool of the oppression and exploitation of one class by another.
Origin
The state is the direct product of the split of society into classes.
A society had just emerged which, by virtue of the general economic conditions of its existence, had had to divide itself into free men and slaves, into rich exploiters and poor exploited; a society which not only could not reconcile these antagonisms but, on the contrary, was compelled to drive them to their utmost limits. A society of this kind could exist only in the midst of an open and incessant struggle of these classes against one another, or under the rule of a third power which, standing apparently above the warring classes, suppressed their open conflicts and allowed the class struggle only in the economic field, in so-called legal form.
Frederick Engels. The Origin of the Family, Private Property and the State, 1884
But the fact that it presents and places itself "above" the class conflict does not mean that it is not part of it. The state is an instrument of class exploitation and oppression.
As the State arose from the need to hold class antagonisms in check, and as, at the same time, it arose in the midst of the conflict between those classes, it is, as a rule, the State of the most powerful class, of the economically dominant class, which, with its help, also becomes the politically dominant class, thereby acquiring new means for holding down and exploiting the oppressed class. Thus the ancient State was, above all, the State of the slave-owners for holding the slaves down; the feudal State was the organ the nobility used to hold down the serf peasants; and the modern representative State is the instrument capital uses to exploit wage labor.
Frederick Engels. The Origin of the Family, Private Property and the State, 1884
The state is not a mechanical, blind tool of the ruling class. It is, above all, a protector of the social order whose main mission is to constrain the permanent struggle that strains the seams of a society torn apart by the existence of classes and exploitation. Its first objective is to hold those seams together in order to ensure the preservation of the order of exploitation and, with it, its own self-preservation. This gives it a certain autonomy even with respect to the ruling class, especially in those periods when the class struggle stagnates without losing virulence and threatens to produce what the Manifesto described as "the common ruin of the contending classes". That is why Engels continues the previous paragraph by pointing out that...
Nevertheless, by way of exception, there are periods in which the warring classes are so balanced that the power of the State, as apparent mediator, acquires a certain momentary independence in relation to both. Such was the case of the absolute monarchy of the 17th and 18th centuries, which kept the balance even between the nobility and the bourgeoisie.
Frederick Engels. The Origin of the Family, Private Property and the State, 1884
Periods of open struggle between two historical exploiting classes, one decadent and the other ascendant, are a struggle for the state, to take control of it, not to destroy it. Both classes are interested in keeping the social whole dominated and cohesive for exploitation, albeit under different social and legal relations. Destroying the state would be equivalent to destroying the conditions under which each expects to build the forms of its own domination.
Before the exploiting classes the state thus appears as "power" itself, and that is why, at least in the moments before and after the great revolutions, the struggle between the new and old masters is preceded and followed by a guerrilla war, a game of positions within the state itself. When the economically dominant class is also politically dominant, the state itself then appears as the terrain of a bureaucratically cushioned struggle between the old class, which resists giving up political power, and the new class, which is waiting to take its place or has already taken it.
The proletariat and the state
For the bourgeoisie as a class, during much of its historical life, the state presented itself as a body to some extent external to itself, precisely because it was necessary for exercising its social domination. That appearance vanishes when the leading role in the class struggle is taken by the proletariat, the first non-exploiting class that aspires to take political power and which therefore...
...cannot simply lay hold of the ready-made state machinery, and wield it for its own purposes.
The centralized state power, with its ubiquitous organs of standing army, police, bureaucracy, clergy, and judicature – organs wrought after the plan of a systematic and hierarchic division of labor – originates from the days of absolute monarchy, serving nascent middle class society as a mighty weapon in its struggle against feudalism. (…)
At the same pace at which the progress of modern industry developed, widened, intensified the class antagonism between capital and labor, the state power assumed more and more the character of the national power of capital over labor, of a public force organized for social enslavement, of an engine of class despotism. After every revolution marking a progressive phase in the class struggle, the purely repressive character of the state power stands out in bolder and bolder relief.
Karl Marx. The Civil War in France, 1871
Forms and objectives of the workers' state
With the entry of capitalism first into its imperialist stage and, almost immediately afterwards, into its decadence, the forms of the workers' struggle are transformed. The mass strike appears, and with it the workers' councils or "soviets", the organization of the class as a whole which, in the course of the revolution, become the form of that "transitional state" through which the working class imposes, that is, exercises its dictatorship, over the logic of capital.
The Soviets of Workers', Soldiers', Peasants' and other Deputies are not understood, not only in the sense that the majority do not clearly see their class significance and their role in the Russian revolution; they are not understood also in the sense that they represent a new form, or rather a new type, of state.
The most perfect, the most advanced type of bourgeois state is the parliamentary democratic republic. Power is vested in parliament; the state machine, the apparatus and organs of government are the usual ones: a standing army, a police and a bureaucracy that is practically irremovable, privileged and placed above the people.
But since the end of the 19th century, revolutionary epochs have brought forward a higher type of democratic state, a state which in certain respects, as Engels put it, ceases to be a state, "is no longer a state in the proper sense of the word". This is a state of the Paris Commune type, which replaces the army and the police, divorced from the people, by the direct and immediate arming of the people itself. Therein lies the essence of the Commune, which has been so slandered by bourgeois writers, and to which, among other things, they have wrongly attributed the intention of immediately "introducing" socialism.
The Russian revolution began to create, first in 1905, and then in 1917, a state of precisely this type. A Republic of Soviets of Workers', Soldiers', Peasants' and other Deputies, united in an All-Russia Constituent Assembly of people's representatives or in a Council of Soviets, etc.: that is what is already taking shape in the life of our country, now, at this moment, on the initiative of a people of many millions who are creating a democracy on their own, in their own way, without waiting for prior authorization, without waiting until the Cadet professors draft their bills for a bourgeois parliamentary republic, and without waiting until the pedants and routine-worshippers of petty-bourgeois "Social-Democracy", like Messrs. Plekhanov or Kautsky, stop distorting the Marxist theory of the state. (…)
The bourgeois parliamentary republic hampers and stifles the independent political life of the masses, their direct participation in the democratic building of the whole state, from the bottom up. The Soviets of Workers' and Soldiers' Deputies do the opposite.
The Soviets reproduce the type of state which was being evolved by the Paris Commune and which Marx described as "the political form at last discovered under which to work out the economic emancipation of labour".
"The tasks of the proletariat in our revolution". Lenin, 1917
With the "semi-state" of the workers' councils, that new form of dictatorship which is nothing but the imposition of the satisfaction of generic and universal human needs can confront the law of value and begin the decommodification of society, that is, take the first steps towards socialism from the first phases of its worldwide extension. That is its essential objective.
The revolution will be a social revolution in the very place where it arises, and not simply a political one (the seizure of power by the proletariat). A workers' state, let us stress, cannot in any case be maintained if the international revolution does not break out, which is why socialism cannot be established in a single country. Let us add that the suppression of capitalist relations is in no way equivalent to socialism (nor to communism!!) but only -and in all this we take for granted that it is a destruction carried out by the working class itself- a step towards socialism. But this step, we maintain, must be taken as quickly as possible through the immediate attack on and suppression of wage labor, even before the proletariat takes power on an almost global scale. Without this, however proletarian the power may be, it will turn into its opposite, since the devastation of the revolution is not a clear and immediately perceptible break.
Even if it occurs in only a single industrial district, a social revolution takes the standpoint of the whole, because it is a protest of man against dehumanized life, because it starts from the standpoint of each real individual, because the community from which the individual strives not to remain separated is the true community of man, human nature. The political spirit of a revolution, on the contrary, consists in the tendency of the classes without political power to put an end to their isolation from the state and from power. Its standpoint is that of the state, an abstract whole that exists only through its separation from real life, and that would be unthinkable without the organized contradiction between the general idea and the individual existence of man. In keeping with its limited and ambiguous nature, a revolution with a political spirit thus creates a ruling sphere in society at the expense of society itself.... Every revolution dissolves the old society: in this sense it is social. Every revolution overthrows the old power: in this sense it is political.
Marx in The King of Prussia and Social Reform. By a Prussian
"Letter to the Third International Conference of groups of the 'Communist Left'", FOR, 1980.