PvE: Boring, scripted, no-skill content made for players who can't succeed at defeating another human. PvE is for when you realize you're absolutely bad at PvP, and you're too lazy to fix it.
PvP: The epitome of skill with a crooked balance. Every comp for the most part has a counter, and it sucks, deal with it. There never has and never will be stable class balance, it's not possible. Classes need nerfs and classes need buffs but at the end of the day, unless RNG really gives you ****, the better players will win.
2v2: My comp beats your comp. This bracket is nothing but team make-up influenced by the skill of the players.
3v3: The most balanced bracket. With the array of possibilites and combinations, but not too hectic and not too simple, this is the main skill bracket.
5v5: Who dies first, this comp is all about whose comp rapes the first player of the other comp.