Gamespot has an update about the gangs in Los Santos. They say that gangs can be identified by the colors of their clothes, tattoos, and finally hand signs. They say that players will have to flash gang signs often to show respect or disrespect. Along with that you'll need to wear the clothes in the same aspect as the signs. Don't want to piss them off.
There are different "sets" within the 4 major gangs (The Orange Grove Families, the Ballas, the Vagos, and the Varrios Los Aztecas) that will either help you or not. For example, in the OGF there are 2 sets (or other gangs) that use to be a part but, because of internal conflict, split when CJ was gone. These 2 gangs are the Temple Drive Families and the Seville Blvd Families. If CJ is in trouble with the Ballas or the cops and is in this area, they will give him a hand. But sometimes, because of the conflict, may not be so much as useful as you'd like.
They also talked about some things that were in the previous games but were sometimes hard to find. That's gang fighting. I'll quote it so you can get the whole information:
"To keep things extra interesting, you'll often see plenty of action, whether you choose to get involved or not, where gang territories border each other. Drive-bys, skirmishes with the cops, and a crew of Ballas jacking some pedestrian's car and going on a rampage through the city are all sights you can expect to see on the streets of Los Santos."
That's pretty much it. Along with these new screens.