In Pokémon GO, there are three different rival teams you can join as part of an effort to capture Gyms: Instinct (Yellow), Mystic (Blue), and Valor (Red). Allied Trainers part of the same team can reinforce their own gyms, and attack/claim enemy team gyms.
Related: POKÉMON GO GYM OVERVIEW: HOW TO CLAIM, DEFEND, AND REINFORCE
When you reach level 5, you choose your team; however, choose carefully because one you pick your team you are stuck.
As of right now, there is no way to change your team once you select it.
There’s no word if Niantic plans on changing this or making it some sort of “pay-to-switch” feature, so choose carefully.
It is also not possible to change what your avatar looks like once you create him/her. Again, we don’t know if this will change in the future.