NFE: Not fully evolved, so yeah pokemon that haven't evolved and are thus unfit for competitive. There are exceptions especially with evolite. Examples of NFE are Gastly, Grotle and Riolu. Examples of exceptions are Scyther and Porygon 2.
NU: Never used, pokemon that are too weak for competitive battling due to stats comparable to NFE pokemon. Examples are Delcatty and Butterfree.
UU: Under used, The favorite tier of the top users here. It contains many strong pokemon that have some fatal flaw that takes them away from OU and BL. Typing, movepool, and stats are usually the flaw. Examples are Torterra (4x ice type weakness), and Ambipom (I think it is stats, he seems decent to me)
BL: Borderline, Pokemon too strong for UU but still not OU. Examples are Medicham (thanks to Pure Power)
OU: Over used, the supposed best of the non-uber best. These even include a bunch of legendaries. This is the Tier right before Ubers. These are pokemon with high stats great movepools and great typing. Many are used to much and are broken, boring, and Sadly predictable. Examples are Garchomp, Weavile, and Dragonite.
Ubers: Ubers(duh), The toughest of the tough these guys are banned from regular play. Most of them are big time base 600-680 base stat legends. It also has non-legends. Examples are Dialgia, Lugia, Darkrai, Wobbuffet, and Kyogre.
RU: Rarely used, A new tier for pokemon that are not used frequently. Some are also in other tiers. These for the most part are decent pokemon. Examples Torterra, Umbreon, and Cresselia.