Democractic Socialists:
Finally, the third category consists of democratic socialists who favor some of the same measures the communists advocate, as described in Question 18, not as part of the transition to communism, however, but as measures which they believe will be sufficient to abolish the misery and evils of present-day society.

These democratic socialists are either proletarians who are not yet sufficiently clear about the conditions of the liberation of their class, or they are representatives of the petty bourgeoisie, a class which, prior to the achievement of democracy and the socialist measures to which it gives rise, has many interests in common with the proletariat.

It follows that, in moments of action, the communists will have to come to an understanding with these democratic socialists, and in general to follow as far as possible a common policy with them -- provided that these socialists do not enter into the service of the ruling bourgeoisie and attack the communists.

It is clear that this form of co-operation in action does not exclude the discussion of differences.
Ok, this is the definition of Democratic Socialists in the Communist Theory FAQ found in this Learning forum. It says that they favor some but not all of the measures which Communists support.

Socialism is a form of class society in which all or at least all of the important means of production are owned and managed by a "central public authority" or state. The people that control the state make up the "ruling class" of such a society.

In such a society, goods and services may be produced for use (freely distributed to the population) or as commodities for sale in a market. Most if not all labor is wage-labor, and surplus-value may or may not be extracted from workers. That's another way of saying that state-owned enterprises may be operated to generate a "profit" or not. The circulation and use of currency (money) prevails. Differences in standards-of-living are normally far less than that which prevails under capitalism...but are still significant.

So far, so good, I trust. Now it gets tricky.

The "communist" regimes of the 20th century actually never claimed to be anything but "socialist". Their argument was that "socialism" was "necessary" as a "transitional stage" to communism.
Now this is part of the definition of Socialism that is found at redstar's site. It says that Socialism is what regimes claimed was necessary to get to Communism. It also says that "most if not all labor is wage-labor, and that the "circulation and use of currency prevails."

Production under socialism would be directly and solely for use. With the natural and technical resources of the world held in common and controlled democratically, the sole object of production would be to meet human needs. This would entail an end to buying, selling and money. Instead, we would take freely what we had communally produced. The old slogan of "from each according to ability, to each according to needs" would apply.
Now this is part of the definition of Socialism at the World Socialist Movement's website. Here is a link that gives 6 points where they disagree with other "Socialist" organizations:

http://worldsocialism.org/introduction/others.html

After reading some of the information at the WSM's website, I wanted to know your opinion about this organization. Much of what they say sounds like it is all Communist rather than only supporting parts of it, as the definition of Democratic Socialism in the Communist Theory FAQ in this forum suggests. And, that being the case, it doesn't sound like a "transitional stage" as redstar defines Socialism. Is the WSM calling itself Socialist just to sound less controversial than if it was the World Communist Movement? I'm a bit confused here. They all have different definitions of what Socialism is. I guess that's why redstar says it's a lot more difficult to define Socialism than it is to define Communism.