Oh baby here we go. I think I can explain this best since I've done some research on it.
The USSR wasn't actually communist economically. USSR means Union of Soviet SOCIALIST Republics. However, it was still run by the communist party.
I've heard quotes saying that the goal of socialism is communism, and I think from an economic viewpoint, communism would have no currency whatsoever, but I know from history that socialism does have currency, so really no nation has achieved communism in the economic sense (as far as I understand).
The best I can understand it as (and keep in mind this is me piecing it together from various sources) is that socialism refers more to economic policy and communism refers more to political policy.
For example, the USSR was socialist economically, as was Nazi Germany. Nazi literally means National Socialist Workers Party. However, they were dead set against each other, and Hitler described Communism as an enemy of Germany (or something like that). But they were both socialist. Germany, however, was politically Fascist (modeled after Fascist Italy) while the USSR was Communist.
To sum it up: Socialist = economic Communist = Political
Again, remember this is what I've pieced together so it probably isn't 100% accurate, it's just how I can best understand and describe it.
Hope it helps!
Caption this Meme
