SJW = Social justice warrior. Basically a subcult of malcontents who complain on twitter and tumblr about racism/sexism/trannyphobia etc, but mostly about how they are treated. One of the comments on that blog summed it up
I think SJ blows up in an unusually nasty way because it combines a felt right to be insulting with a felt right to not be insulted. Throw in a theory of microaggressions, no sense of scale (everything bad is permitted to be treated as equally bad) and a willingness to assume that any pain caused is a result of bad motivations, and you get a rage machine.
All of these goofy hashtags, like #yesallwomen and #CancelColbert come from SJW's. And their favorite thing to do is to shout down white males who think of themselves as liberal. A lot of them have some background in leftist humanities (i.e. gender studies, african -american studies, etc.) so they like to redefine words (e.g. racism isn't hating people from other races, it's privilege + bad treatment, so saying "I hate whitey" isn't racist; or "violence" means "anything that upsets me, like namecalling")