For Marx (and for the soviets), communism is a potential economic form. It is characterized by common ownership of the means of production and... that's pretty much it. That's Marxist communism. Socialism is a way to get there and socialist economic elements can exist alongside capitalist ones. Under capitalism, land, machinery, technology, and knowledge are all owned by a few people and profit is the driving factor in determining what is produced. We've experienced a bunch of different forms of capitalism, from Keynesianism to neoliberalism. These differ on the role of markets. Socialism comes in many forms, the same way that capitalism does. Associating communism with Stalin I think is a very American viewpoint, a relic of the cold war.
Other places have very different conceptions of communism. In Kerala the Communist Party of India (Marxist) were recently in power and brought in land reforms, and education reforms, and the female literacy rate is now the highest in India. Meanwhile the Naxalites (maoist guerillas) are occupying the middle of India. So modern Indians have a much different idea of communism than we would in North America (or Europe, western world in general). There have been a lot of states in modern history which have had 'actually existing socialism': Cuba, Vietnam, Venezuela, China, and North Korea off the top of my head. Most of these haven't lasted as socialist because of international intervention, especially from the USA. North Korea for instance, was a workers state supported by the USSR until the Korean war. After the country was totally dessimated by the US army it lost a lot of its resources, became much more concerned with its protection, and became a dictatorship; this worsened after losing the USSR as its only ally. Vietnam's communism disappeared after the Vietnam war. South American communists and socialists have trouble
A lot of Marxists (especially Trotskyists) feel the need to distance themselves from Stalin, to go out of their way to condemn Stalin. I don't really think this is necessary. Stalin was horrible and violent but his actions I think say a lot more about the dangers of state violence than about communism. The same way that studying Robespierre tells us very little about liberal democracies. The USSR was originally formed as a dictatorship of the proletariat, and then a dictatorship of one dude. The main charateristic or soviet socialism is a centrally planned economy.