Um, explanation for those of us who have NO IDEA what that is?
Monotype is where you can only use pokemon of one type, but they can be dual typed into something else. For example Gastrodon is a Ground Water and is legal for the Ground monotype and Water monotype. Everyone on my team has the Ground typing.
The website is a way to play pokemon online with a better system then the one in the actual games and allows you to use any pokemon ever. It also has Teir functions and tournaments.
Oh yeah, and OU stands for over used. My only Over Used being the Dragon.
The teirs are:
Uber
Over Used
Under Used
Rarely Used
Never Used
Uber and Never are the only power based teirs everything else is how much people use them. Ubers are for almost Over powered pokemon and Never used is for horridly weak pokemon that will be killed easily. Like Charizard and Magikarp for instance.