1.Gta 5 is an open world game where you can do whatever you want (of course with some limits in the game). You can get cars and customize them and do missions for different people. The three main characters are Trevor, Franklin, and Michael. You can get cars and customize them, and fly planes , and helicopters. You can also just free roam and enter cheat codes to make the game for fun or to simply complete a mission. There is boats and a realistic ocean. You can buy properties as well.

2. Forza motorsport 4 is a racing game. You can customize cars and buy new ones. You have a rank and when you level up you get to pick a new car. There is many custom paints and decals you can buy. You can do simple races or even play car bowling.

3. They are similar because you can customize cars with different paints, and upgrades that make your car go faster or have better handling. You can do races in both games.