My understanding is (and I could be wrong on some details, I’m no political scientist!), communism is a broad idea for the end goal of a society with no class boundaries, no private ownership, workers (or a representative for them, in some models) owning the factories/“seizing the means of production”, rights for all, everyone gets their fair share.

There are many, very different ideologies for achieving such an ideal society. Additionally, different people have tacked on their own ideas onto it (such as the necessity of a violent or peaceful revolution, how to redistribute land, etc.). You can go from forms of anarchism (very anti-authoritarian) and democratic socialists (usually quite anti-authoritarian) to Stalin’s USSR and the Eastern bloc during the Cold War (very authoritarian with cruel dictators at the helm). Like most belief systems, there is plenty of infighting between various different factions.

“Socialist” is another broad term and is usually used to describe groups, people, and governments that implement policies that will build up towards the communist ideal. It’s thrown around for many democratic groups as well as deeply authoritarian ones, hence the separation between authoritarian and anti-authoritarian communists that is very common.

You might have heard of terms like “democratic socialist” and “social democracy”. The former are socialists who usually advocate for slow reform over a revolution, the latter are capitalists who implement socialist elements in their policies. Both try to uphold liberal democratic processes and are against one-party states like that of the USSR.

TLDR:

• “communism” is the ideal and optimistic goal for a state, people who pursue that are called “communists”. It is an incredibly broad term that can describe very different ideologies
• "socialism" is used to describe groups that implement policies towards the goal of communism, people who support this are called “socialists”
• “democratic socialists” are socialists who support liberal democratic practices and usually advocate for slow reform rather than violent revolution
• "social democracies" are people who support some degree of socialist policies in order to make society more equal and fair while retaining the capitalist system
• the separation between authoritarian and anti-authoritarian communism is very important! They are vastly different despite their shared goal of a classless society