What in the world is and OU?!
Fine. I'll Explain
OU:Overused. Because of their movepool, stats, and/or abilities, these pokemon are very strong, can last in battles, and/or cripple teams not prepared. It doesn't necessarily mean EVERYONE uses them, rather its their rank in power.
Off the top of my head, here are a few OU's.
Salamence (He's so strong, he has multiple roles-Physical Special, or Mixed)
Weavile (Strong, Fast, & able to rip teams to shreads)
Bronzong (A great wall, & very hard to bring down. Unless you are prepared, get ready for a long, drawn out match.)
Tauros (Despite his small, limited movepool, it uses what it has VERY well, especially with Choice Band.)
BL: Borderline. These pokes are stronger than [most] UU, but aren't strong enough to consider them OU.
Some examples are:
Floatzel (His movepool is rather limited for a sweeper, but it can cause pain)
UU: Underused. These pokes usually have something to bring them down, such as poor movepool, bad stats, or, quite simply, an OU can do what they can, but better.
Some UU's are:
Vileplume (My personagl fav, I use him on my team. He is still UU, because of his very limited movepool-mostly Grass & Poison)
Blazekien (Quite simply, Infernape is an improved version)
NU: Neverused. These pokes, quite simply, sucks. Alright? They suck. I cant break it down further than that.
Delibird (Poor movepool, pathetic stats, and what the hell is the point of Present?)
Of course, UU can indeed stand a chance. I mentioned I use Vileplume. Despite his poor movepool, he has great stats. Hidden Power gives him some diversability, he has a WIDE array of crippling moves, & he is one of the FEW pokemon able to pull off Sunnybeamer. His typing opens him up to many weaknesses, but it opens up to many resistances, most are in the OU game.
Its all about how you use them.