I. What is an SJW?
For me and what I've observed in social media are radical leftists. They're often leaning more towards emotions rather than scientific facts. As extremists. As stated prior, they're a minority, however, they're loud in social media such as Twitter or Tumblr. They tend to isolate outside information and cater to someone's feelings. Make them feel they're fighting for justice and a fair world for 'everyone'. In short, similar to a cult following.
II. Have You Ever Actually Met One?
Yes, I've on multiple occasions. I've two friends, for anonymity sakes, I will give them false names. Andrea is the main problem. She has made many issues between my friends and my boyfriend. Andrea thinks only her opinion is the only right one and if one tried to counter with actual facts, she will immediately start rising her voice, give racist/sexist etc replies. My other friend, Tea, would string along with Andrea but she would see common sense where Andrea doesn't. Andrea has also said multiple racist comments in front of me regarding Mexicans while in the same sentence claim she is for minorities (I'm myself, half Mexican).
We were a group of friends playing Dungeons And Dragons (a board game) and Andrea and Tea would bring problems from outside the game into the game.
It got so bad my boyfriend had to talk to my teacher at school regarding the issues.