http://www.smogon.com/bw/tiers/ Here you can look at tiers. Different Pokemon are in each tier, for example, Legendary Pokemon are likely to be in the Uber tier because of their massive stats. Here is a link to each tier so you can see which one to check out first:
Uber: http://www.smogon.com/bw/tiers/uber
Overused, with "Overused Pokemon". These are all those "I want that Pokemon!!".: http://www.smogon.com/bw/tiers/ou
For borderline, credit to JCM! (Muslim Bro xD)
Borderline: BL2 and BL are Pokemon that are too strong for the previous Tier but not strong enough for the next tier. For example, BL Pokemon are Pokemon that are too strong for UU but not strong enough/ limited use in OU.: http://www.smogon.com/bw/tiers/bl
Borderline 2: BL2 and BL are Pokemon that are too strong for the previous Tier but not strong enough for the next tier. For example, BL Pokemon are Pokemon that are too strong for UU but not strong enough/ limited use in OU.: http://www.smogon.com/bw/tiers/bl2
Underused, with underused Pokemon. As the name suggests, they are underused. : http://www.smogon.com/bw/tiers/uu
Rarely Used, with rarely used Pokemon. You rarely see these Pokemon in people's teams: http://www.smogon.com/bw/tiers/ru
Never Used, Pokemon that are never used are in this tier. These are the Pokemon that you see and say "Meh, I don't want these, i want Tyraintar.": http://www.smogon.com/bw/tiers/nu
Little Cup, for Pokemon at their first evolution. Most of these are those where you go "Aww! So cute!", and don't want to evolve. It's those at the first evolution: http://www.smogon.com/bw/tiers/lc
Limbo is for those special Pokemon without a home in a tier: http://www.smogon.com/bw/tiers/limbo
Not Fully Evolved, usually for second evolutionary stage Pokemon. Some see it as a Little Cup upgrade: http://www.smogon.com/bw/tiers/nfe
Edit: I made it easier on the eyes and added more info.
