Chop is Franklin's dog in Grand Theft Auto V. He's a Rottweiler and it is currently unknown how you will be able to interact with him in-game, however a pre-release screenshot reveals that he will be able to sit in the car with Franklin, and possibly occupy a seat like other NPCs. In Franklin's Trailer, Chop is shown to be attacking a Ballas gang member; leading to the conclusion that he can help Franklin during fights. However, it is unknown if other dogs will do the same. Chop can also help the player to find hidden packages.
Update ~ Chop can be customized (to some extent) you will be able to change Chop's collar colour, but more worryingly there is the potential for Chop to be lost, furthermore Chop can even sadly die. So if you don't want to be emotional, keep Chop alive as long as you can!