Rockstar Games explains in a recent blog that they have “implemented [several] levels of difficulty into the game”. Mostly, the change appears to be in the game’s use of the auto-aim feature.
“Casual mode gives you a full auto-aim system (and faster regenerating health). Normal mode features snap-to-target with precise aiming using the right stick. Expert mode implements free aim with no lock-on at all.”
This would be Rockstar’s first foray into providing multiple difficulty options in their open-world games (no Grand Theft Auto game has ever had the option). We will have to wait and see how successful their risk pans out, but this route should definitely attract those that may find the game either too challenging or easy.