Tiers vary from server to server, both in the structure of the tiers and the Pokemon assigned to each one. The following are Reborn's tiers, the structure of which is a fairly common layout:
Ubers- These Pokemon are banned from all others, and are generally regarded as highly over-powered.
OverUsed (OU)- This is considered the tier for standard play. These Pokemon are considered the best, without being too strong for use.
Borderline (BL)
UnderUsed (UU)
LittleUsed (LU)
NeverUsed (NU)
Reborn's tier list can be found
here.