As I understood it, The main diffrence was communisim came by force, likely from a cival war. While socialisim was the gradual changing through goverment laws and such to a communist state (same as communist, but without violent war)
The Spectrum of Capitalisim (United States) --- Socalisim (Spain) --- Communist (North Korea) is a good definition.
Interestingly enough, while some predicted Communisim to be Capitalisims equal, History does show that Capitalisim currently is the most stable form of government. The biggest problem with Communism is corruption in the system, something nearly unpreventable. The next is the lack of motivation for workers to work, After all, Joe Shmo gets exactly the same benifits (If the system is not corrupted) as John Doe. And Joe could be a ditch digger and John a Nuclear Physisist.
One day, I think communisim might be the end result, but that can only happen after A LOT of reform civilization as we know it, People will have to care for one another MUCH more then right now, to the point where they put the needs of others infront of their own. This is the major change that has to happen in a society that is required before I think the communisim will work well.