I'm basing this on Marxist theory, I haven't studied other theories of communism. Communism, is the stateless, and classless for of society. Everyone is happy, and equal... blah, blah, utopian. Socialism is the stepping stone to this, where the state regulates everything. The USSR was socialist, hence the name: Union of Soviet Socialist Republics.

Communism is utopian, and socialism has been tried and failed. I believe social democracy to be a good form of government, but like all forms, it's not perfect.

For an explanation on communism.
http://en.wikipedia.org/wiki/Communism