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Mods, or add-ons, are modifications to the game.
download it then chuck its contents in a mods folder in minecraft .jar
look what ever game you want and add the word mods to the end on google. e.g oblivion mods, GTA mods
Suggest you have a look at these sites to see if they can help you:
gtaivmods.com/
www.moddb.com/mods/gta.../how-to-install-mods-in-gtasa
Yes. Optifine can count as a mod. Just place your optifine.jar in the mods folder, and add other mods with it.