Essentially, radical social justice viewpoints taken to absurd extremes. Sometimes to the point of expressing highly racist, homophobic, or transphobic views in the course of supporting some other group they feel is even more persecuted than those groups.
Now, the term is being thrown around by some people to describe those who are left leaning and support a viewpoint they disagree with.
So-called "Social Justice Warriors" are indeed an actual and rather ridiculous (yet amusing) group on the Internet, mostly centered around Tumblr, but their overall population density is pretty low compared to progressives and feminists in general.
Some examples of the second group might include the people who flagkilled half of my comments in this thread. Man, I know political threads on Reddit News are a steaming pile, but y'all could at least pretend to be capable of talking to people you don't agree with and downvote me into oblivion instead.
I realize it's tangential to your point, but I'm curious how would you define or describe a SJW?