Yeah, I agree with the posts above about how even in the English language nobody ever seems to reach a clear consensus. For those who actually have a solidly accepted definition in their own language (for the equivalent words) I'd be interested to hear it as well.
I generally prefer to use communism to refer to being very left in general, just as capitalism is right. "Public ownership of the means of production" is a pretty simple definition I've had since grade school so that's a start. Thus, communism can refer to a wide spectrum of governments/societies all along social or other non-economic issues, just like I'd use capitalism.
What I really hate is people who assume or assert communism = authoritarianism: I tend to want to very clearly state that this is not the case, any more than capitalism = authoritarianism because states like Congo could also be defined as "capitalist." If you wanted me to give a somewhat simple division of ideologies that explains this, it would start with distinguishing socialism and Stalinism/Maoism/Leninism as both under the heading of communism. In short:
Authoritarian/Dictatorial communism = Stalinism
Democratic/non-authoritarian communism = socialism.
It parallels the use of capitalism a bit but as an economic system I think capitalism has existed in a wider variety of states otherwise -roughly something like:
Authoritarian capitalism = Fascism or Theocracy, or just a Dictatorship
Democratic/Non-authoritarian capitalism = Libertarianism, or just Anarchy
So with capitalism this is not to say that capitalism alone makes a state one of these - A state that otherwise fits some other label, could have communism or capitalism on an economic scale. These are also extremes in every regard - there can be centrists on both the economic scale (between communism/capitalism) and anything in between on the other dimensions as well.
Finally, I grudingly accept people who use Communism, Capital C to describe states like Stalin's or Mao's as authoritarian, but then the whole invention of a term like Stalinism is wasted. If communism just means "left - as in control of economic means of production" it can applied to different situations without carrying as much stigma.
In pretty much all cases, though, socialism means economically left with a progressive society that preserves social liberties, and very often includes democracy. Anyone giving alternate definitions of socialism in English is being even more stupid (there's always a few exceptions - like maybe talking about communal tribal structures historically but not in modern context). This includes idiotic claims about "National Socialists" or the idea that government simply helping out big business is "socialist" (tough call on what it is but use something like plutocracy or just plain old corruption).