Ubuntu is a flavor of Linux (called a distribution, or "distro")
You can think of Linux distros like a family tree- they all have the same common root, but the many distros have all been made to achieve certain goals or ambitions. And there are offshoots of offshoots- Ubuntu is based on an older distro called Debian. Ubuntu itself has its own little offshoots, like Mint, Peppermint and Bodhi. Debian is still very much around and still being used as a base for new distros.
Linux distros generally operate on the same basic concepts but really can be very different in how they accomplish things.
so in fact it is linux but it is used for a specific purpose?
I want to know want is the best linux for gaming?
Yes, it's Linux. You can read about what Cannoical's goals in making Ubunutu were on the Ubunutu site.
As for gaming, here my general answer:
No Linux distro is good for gaming. If you want to play PC games (ala Steam) stay with Windows.
More detailed answer:
CAN you game on Linux? Sort of. Does Steam run under WINE? Yes. Do Steam games run under WINE? Sort of. The older the game the better your chances.
Even under the best of circumstances the games aren't going to run terribly well, and usually will require a fair amount of technical sweat equity from you to research fixes and try to get things to work.
I think gaming on linux is better viewed as "for the sake of it" or "to see how far I can get", and not as an actual viable alternative to Windows.