Its a Mexican gang in LA started in a place called Florencia. Roughly 2,000 members mostly in Souther Cali and Arizona/New Mexico
The 13 stands for the letter "M" which means they are an affiliate of the mexican mafia - as M is the 13th letter in the alphabet
They also go by F13 for short and normally mark there territory by graffiti with arrows and F13. it can be observed when in the Florencia are of Cali at the back of Liquor stores and street corner walls
Recently, as of 2007, they have been in the news for killing random black people (innocent) as they did not know if they had gang affiliations. It has created alot of racial tension - particularyly in the prison population as seen in 2007's black/mexican prison riots.
Hope peace and understanding come soon.
Homy #1 - Ay holmes, Florencia 13 for life.
Newsguy - Three Florencia 13 gang Members were arrested today in los angelos