Communisms become an unfortunately loaded term, originally there was little to no distinction between the 2, hence why Marx so freely used the term communist, while hardly being an authoritarian.
The only real distinct difference was in Leninism which usually referred to a workers state as socialist and a classist state as communist, of course that all got very muddled, and the terms were still largely used interchangeably.
"Communism" eventually got linked with the eastern bloc and china, and socialism with western leftists.
Left-communism and Euro-communism etc, as well as anarcho-communists/socialists have very little to do with the communism of Stalin and the USSR, but it would be disingenous to say they're not communists.
Basically Communism and socialism used to mean the same thing, now they don't because stalin fucked that all up for everyone.
When I refer to myself as a socialist I'm being very broad, I have mass contradictions even with my own statements, due to inherent differences in different socialist thought, socialist is just an easy label, I don't neccessarily agree with many socialist groups on very key points.
Probably the reason this is so common amongst leftists:
Edit: just to make it clear
British socialist parties:
Socialist Workers Party
Socialist Party Of Great Britain
Communist Party of Britain
Communist Party of Britain (marxist leninist)
New Communist Party
Some of those are members of TUSC (trade union and socialist coalition) but still...