You'll see the PlayStation 2 logo and your game will begin. If your game is compatible with your PS3 model, it will start just like a PS3 game.
You can check the full list of games and what models they are compatible with here. This is more common if you have a CECHCxx (60 GB) or CECHExx (80 GB) model, which use software emulation instead of full hardware backwards compatibility. While you can usually just pop the PS2 disc into a compatible PS3 and start playing without issues, some PS2 games have known compatibility problems.
You may run into issues with some PS2 discs. CECHCxx (60 GB) and CECHExx (80 GB) - Limited software emulation.CECHAxx (60 GB) and CECHBxx (20 GB) - Full hardware backwards compatibility.The last digits will inform you as to whether you have full hardware backwards compatibility, or limited software emulation: Find the sticker on the back of your PS3.
If it has two USB ports, it cannot play PS2 discs. If the PS3 has four USB ports, it is backwards compatible. If you have a fat PS3, check the number of USB ports on the front of the PS3. All backwards compatible PS3s are "fat," but not all fat PS3s are backwards compatible. Check the number of USB ports your fat PS3.Doing so will void your warranty and can get you banned from the PlayStation Network. You can jailbreak a PS3 to play PS2 games.If you don't have a backwards compatible PS3, the only way to play PS2 games on it without jailbreaking is by purchasing and downloading the games available on the PlayStation Store.The ones with 4 USB ports on the front are. The "slim" and "super slim" models are not backwards compatible. Only some fat PS3s are backwards compatible, but not all of them are.
The original PS3 design is often referred to as the "fat" PS3.