Karl Marx's alternative to capitalism is communism. This is the system that he thinks should (and eventually will) arise to replace capitalism.
Marx believes that capitalism is a system that inherently exploits the proletariat. Therefore, he argues that a capitalist system will always be marked by conflict between that class and the bourgeoisie. He believes that this conflict will inevitably end with the triumph of the workers.
When this is fully accomplished, there will be a system in which there are no classes and no private property. People will all work because they want to, not because they hope to make profit. Everyone will receive the material goods that they need so there will be no poverty. There will be no bourgeoisie who will take the money that is made by the workers. This will be a utopia in which everyone has what they need and there is no longer any class conflict.