These are all competitive tiers and formats. All of these tiers and formats have different rules and different sets of Pokemon you are allowed to use.
- VGC stands for Video Game Championships, which usually refers to battles that would be equivalent to how the game structures it's own ranked battles.
The tier system is different, however, and completely fan-made. They vary in Pokemon viability, and are essentially just a way to rank the usability of Pokemon.
- Ubers is on top, (it means "above competition" in German), the best of the best Pokemon are restricted to Ubers
- OU stands for "Over-Used" and is the second highest tier
- UU stands for "Under-Used*
- RU stands for "Rarely-Used"
- NU stands for "Never-Used"
- PU stands for itself
- ZU stands for "Zero-Used"
AG should also be mentioned, another incredibly popular format that stands for "Anything Goes". In this format, you can use any Pokemon from any tier, and you can even use multiple of the same Pokemon.
Long story short, other than VGC, they're just a bunch of unofficial tiers and competitive formats.
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