In short, it took all the best elements of Grand Theft Auto: San Andreas, and multiplied them by absurdity.
To give you a vague idea, "Strap it on" is the tagline, and my sweet husband has taken to running around beating rival gang members with a weapon called the Penetrator. Given the nature of these forums, I'll leave it to you to look it up for yourself.
The game's opening sequence is that you are robbing a bank DRESSED as one of your gang's most famous members, with an actor in tow. As you might expect it becomes a cluster... um... frak, and the next thing you know, you're hanging off of a massive vault that has been hauled out of the bank by a crane helicopter, firing an automatic rifle one-handed, to take down enemy helicopters as you make your getaway.
There's one chase sequence in the game where you are riding a rickshaw pulled by a gimp (yes, THAT kind of gimp). Amusingly, even the rickshaws explode when they take enough damage.
"Activities" include everything from base jumping, car surfing, stunt flying, car jumping and streaking to destroying rival gang operations and participating in a crazy Japanese-inspired "reality show" called Professor Genki's Super Ethical Reality Climax.
The fact that you can play 4-player coop throughout the entire game is a huge bonus. It' s all about mindless, absurd, fun. "Realism" be damned.