The Uber tier is effectively a banlist for the OU tier, as "standard" battles include everything in the OU list and below. Pokémon in the Uber tier were deemed too powerful or destabilizing to the OU metagame to allow. Ubers, if allowed, cause extreme "centralization" of strategies—in other words, it is impossible or very difficult to counter them in a reasonable manner.
The Uber tier may include any Pokémon, not necessarily just Legendary Pokémon. Typically, it includes all Legendary game mascots except Black Kyurem, due to various factors preventing it from becoming "too centralizing". Although many legendary Pokémon reside in the Uber tier, a far larger number exists in lower tiers due to similar inadequacies.
After the release of Pokémon Omega Ruby and Alpha Sapphire, due to the dominance of Mega Rayquaza, Smogon made their version of Ubers a full-fledged tier, allowing bans from the tier. Smogon has also banned the move Baton Pass from their version of Ubers. However, unlike other tiers, it is not affected by usage. As a result, Smogon also endorses a playstyle referred to as "Anything Goes", which has no fan-imposed rules other than the "Endless Battle Clause".
OverUsed, commonly abbreviated as "OU", refers to Pokémon that are most frequently used in standard play.
OU status does not have to do with a Pokémon's power or ability. The Pokémon that make up OU, as with all tiers, are determined based on usage. A Pokémon's tier may change from generation to generation. For instance, Magneton is often considered OU in Generation III because of its advantage over Skarmory, another commonly used Pokémon. In Generation IV, Magnezone, the evolution of Magneton, took Magneton's role in OU in some tier lists due to its generally higher base stats. If an OU Pokémon is not used enough in the OU tier, it will fall to the UU tier.
UnderUsed, commonly referred to as "UU", consists of Pokémon that are not used enough to be in OU. Pokémon classified as UU are often outclassed by Pokémon in higher tiers, possibly as a result of generally lower base stats, available moves, or Abilities. For example, in Generation V, Machamp fell to UU in some tier lists largely as a result of the introduction of Conkeldurr, which has Mach Punch, Drain Punch, and generally superior base stats. UU Pokémon are commonly used in OU battles, but most have smaller niches. If a UU Pokémon is not used enough in the UU tier, it will fall to the RU/LU tier.
RarelyUsed/LittleUsed, commonly referred to as either "RU" or "LU". While the name of the tier varies, its "level" is equivalent. The tier consists of Pokémon that are not used enough to be in UU. For example, Escavalier is considered RU in Generation V in some tier lists because it is not used enough in UU battles. If an RU/LU Pokémon is not used enough in the RU/LU tier, it will fall to the NU tier.
NeverUsed, commonly referred to as "NU", consists of Pokémon that are not used enough to be in RU/LU. The name is not literal, stemming from the tiers of Generation I, where Neverused Pokémon had generally lower "competitive value" in comparison to other Pokémon. Some Pokémon that are pre-evolutions of Pokémon in higher tiers may be classified as NU.
PU consists of Pokémon that are not used enough in NU. Many Pokémon in PU are not fully evolved and/or have lackluster stat distribution. Unlike the other tiers, PU is not an acronym, but rather stands for how these Pokémon "stink" compared to higher tiers.
Not Fully Evolved
This group is more commonly known as "NFE", and as the name implies, contains most Pokémon that have not yet reached their final evolution stage and do not have a high enough usage to appear in any tier. This group is not actually a tier, but rather a collection of Pokémon without significant usage even in the lowest tier.
I got this from bulbapedia, so dont blame me, and read the descriptions before you comment.