Teams

Currently there are 3 teams in-game:
There doesn't seem to be any difference between them apart from color, a short story about their views, and a leader.
 * Team Mystic (blue)
 * Team Valor (red)
 * Team Instinct (yellow)

Each of the team symbols are depictions of the three legendary birds in the Pokémon universe:
 * Articuno (Mystic)
 * Moltres (Valor)
 * Zapdos (Instinct)

'You must be at least level 5 to join a team. Tap a gym after level 5 to be prompted to join a team.'

After choosing a team, you are bound to that team, although there are rumors of this changing in the future.

Team members can work together to neutralize and claim enemy gyms, or level up and protect friendly gyms.