Teams
In Team Fortress 2 Classic, two (sometimes four) companies compete against each other: RED (Reliable Excavation Demolition) and BLU (Builder's League United). These companies make up the two teams the player can play (In Non-Four-Team Play). There are also two more teams, GRN (Global Radio Network) and YLW (Yard Logistics Workers), which a player can join if they are playing in a Four-Team Map. An additional "team", Spectator, also exists in which a player can join if they wish to spectate the participating players.
Backstory
TODO, GRN and YLW play a part in all the broadcast towers and freight yards RED and BLU have been fighting over, respectively. Still, they've also got shady corporate interests that they're willing to hire mercenaries to protect.
In-game
During a game, players work with their team to fulfill the objectives of the game mode they're playing. In standard attack-defense gamemodes, RED is the defending team and BLU is the offending team.
Some classes have weapons that are designed to interact with a player's team, for example, the Medic's Medi Gun is used to heal teammates and the Civilian's Umbrella, once ready, grants a teammate mini-crits.
While the objective of most games is to help your team, certain players will ignore the objective or combat, and play as Friendlies (A pseudo-support class to both sides.)
Team Selection
TODO
Classes
A team can consist of a maximum of 32 players on Two-Team, and 16 players on Four-Team, each being one of nine main Classes. Each class is designed to be functionally different than the others and has weapons, to fulfill a particular role within a team. All of the main nine classes are split into 3 sub-groups: Offense, Defense and Support.
Spectater
In Spectater Mode, a player can watch the match through participating players' eyes and fly around freely. While respawning, a player will be in spectate mode (but will not be listed as part of the Spectate team) until that player respawns.