Communism is an eventual end goal, a classless, stateless society. Socialism is a system that aims to progress towards that goal.

These terms have become muddled due to social democrats dropping the pretense that they had any aims at establishing communism (early social democrats like Eduard Bernstein argued for using reformism to establish communism), while still holding on to the “socialist” label. So there are some people who would use “socialist” to describe social democracy and reformism while reserving “communism” for Marxist-Leninists. This is quite strange considering that it was called the USSR and not the USCR, but what are you gonna do?