Ou and Über are tiers used by competitive battlers that organize Pokemon by their competitive value based of their usage and whether they are op (overpowered).There are typically six main tiers for single battling:
Über: the Pokemon banned to this tier are the best of the best. They are so powerful tgat they were banned from standard play. A lot of legendaries are found in this tier, but also some non legendary Pokemon, such as blaziken and excadrill, can be found here.
It should be noted that all Pokemon may be used in this tier
Ou (overused): this is the next highest tier, and typically the one most competive battlers use. It contains many powerful Pokemon, both legendary and not, such as lucario, haxorus, and latios. No Pokemon from the uber tier are allowed in ou battles.
BL: this isn't a tier but a ban list. No Pokemon from here are allowed in Uu and below.
Uu (underused): this is the "middle" tier, containing mostly above average Pokemon, such as scrafty and bronzong. No Pokemon from ou bl or ubers are allowed in uu battles
BL2: another ban list, containing Pokemon deemed to powerful for ru.
Ru (rarely used): this is the "average" tier. No Pokemon from uu BL BL2 ou or ubers allowed.
NU (never used): this contains the "below average" Pokemon. No Pokemon from the above tiers are allowed
LC (little cup): this is a standalone tier. It is for unevolved Pokemon only. Some, like Scyther, gligar are banned.
This should help you out. It's an introduction to smogon tiers, the most commonly used tiers, the ones I mention above. Hope this helped.