PlatinumDude...you really didn't explain it well XD
Basically, Pokemon games are always released in pairs this is why there are two games. Story wise there is usually barely anything different between the two games so if you get X, it'll have the exact same story as Pokemon Y with a few minor differences.
These minor differences are what we call "Exclusives". As PlatinumDude said, if you were to get Pokemon X you can catch Swirlix, Clauncher, and Xerneas in the game, while if you get Pokemon Y you can catch Spritzee, Skrelp, and Yveltal in the game. These so far are the only exclusives shown.
The reason why there are exclusives is because Gamefreak, the creators/developers of the game series, push a lot of emphasis on its players interacting with other people. So you can trade your exclusives for their exclusives.
When deciding upon a game, version exclusives are usually the selling point since the games are exactly the same thing.
For you, since you are new, try to learn what you can about each Pokemon that is excusive to each game and this should help you learn which game you want to get.