Though Vv didn't give a detailed answer, there is more than enough Here
anyway they are tiers that separate the strong from the meek.
They can be determined their tier by stealth rock weakness, bad move pool, typing, Pokemon in higher tiers, def, offence and other reason obviously the best legenderys ate ubers but abilitys are also another factor.
Lc is differance as all Pokemon are first evolution lv5.
Pokemon in ou can't be used in ou because it is a higher tier but uu Pokemon and below are aloud in ou and this is the same system for all tiers.