Class struggle refers to the opposition and struggle between opposing social classes. It is the manifestation their opposing fundamental interests and is the basic means of resolving the contradiction between an existing class structure and the developing forces of production and relations of production.
The essence of class conflict is the opposition between the economic status and material interests of different classes. All class struggles occur on the basis of opposing and conflicting material interests, that is, economic interests, and are ultimately centered around material interests.
In a class society, the class struggle first takes place between the two basic classes in each social form, such as the struggle between the peasantry and the landlord class in feudal society, and the struggle between the proletariat and the bourgeoisie in capitalist society.
Though it is a common misconception, the doctrine of class struggle is not an original Marxist idea. Vladimir Lenin in the State & Revolution made it clear that class struggle has existed before Marx. What makes class struggle inherently Marxist is when the extension of this recognition is made to the dictatorship of the proletariat.