A SJW is someone who thinks they know how to right all the world's wrongs.
The term is pejorative because one or more of the following implications usually apply:
-- The SJW has hubris, being blind to any possible unintended consequences of their "solutions"
-- The SJW cares more about "nice feelings" than good policy
-- The SJW is a generally ignorant of historical, governmental, and economic context
The SJW is usually a shallow, utopian thinker-- in that way they share a lot in common with anarchist libertarians.
Usually a ultra liberal aka a progressive
It's better to leave these terms out of it; they are too slippery.
For example, I am a supporter of universal healthcare-- that makes me a progressive (on that issue) and, depending on how you want to define it, "ultra-liberal" as well, since my position is to the left of even most mainstream Democrats (once again, on this issue).
On the other hand, in the majority of countries around the world, for example, the UK and Canada, universal healthcare is taken for granted even by conservatives... so the same belief that would make me a "ultra liberal progressive" in the US fits with being a mainstream conservative there.
So, the terms liberal and conservative are issue and context specific; whereas, SJW is a kind of overall worldview.