Here is standard for Pokecharms, Most sites/people play mostly by these rules, but there are also Tiers. Each pokemon fits into a specific group. OU (Over-used), UU (Under-used), NU (never used), and Uber (Mostly legendarys.)
Here is a list of Tiers so you can see what Pokemon are in what Tier. So if someone says, I want a UU Battle, you'd battle with pokemon only within that Tier. There are also other ways to battle. Lvl 50, Lvl 100, Open Level, singles, doubles, 3 on 3, and more.