"Black Lives Matter" has become a mantra for people protesting police violence against African-Americans.
It's a hash tag, a popular T-shirt slogan and a movement that is loosely organized — by design.
Black Lives Matter was founded by three women who popularized the slogan during protests over the 2012 killing of Trayvon Martin, an African-American teen who was shot by George Zimmerman, a neighborhood watch volunteer in Sanford, Florida.