PU to put it simply is an unofficial tier made out of Smogons least used NU Pokemon.
PU (What does this even stand for) are the Pokemon that even NU doesn't want, basically Pokemon that have less than 3.41% usage in NU are technically "PU". In PU, some Pokemon - otherwise regarded as bottom of the barrel - could become top tier threats. With the Pokemon ranging from lovable Luvdisc to the raging Rampardos, it's your job to find out which Pokemon and strategies dominate the tier (like VolTurn in OU).
EDIT: PU is now an Official tier alongside the other Smogon tiers.
Here is an example team