JRK:
A "social justice warrior" is a term used for a particular type of advocate for feminism. I don't know who invented the term; it is used by some in a highly derogatory fashion to mean a "crazy feminist" and by some to mean something positive ("a very engaged feminist").
The typical person who would be called a SJW is someone who believe that the patriarchy is a real thing influencing in a very negative way the lives of women (and men), that we have a systemic rape culture and who often subscribe to marxist ideas as well as postmodern notions of gender and gender identity. Note there is a huge range of opinions and persons that fall under the umbrella term.