social justice: promoting respect and equity upon all people of all minority and majority demographics
SJA: short for social justice activist/activism/awareness - promoting social justice with the specific goal of educating people to be more equitable and respectful of all people regardless of demographic (or, someone who does all this). Best taken as a compliment.
SJW: short for social justice warrior/wanker - someone who uses social justice and social justice activism/awareness to bully, harass, and dogpile people, often over semantic mistakes as much as legitimate offense, refusing to accept apologies or acknowledge mistakes for being just that, and general using SJ(A) to be an asshole. As you can probably tell, it's generally not used as a compliment.