6v6 is a competitive format played in Team Fortress 2. It is played with a teamlimit of 6 players per team.
Since the beginning of the competitive scene, many experiments with different formats were made, 6v6 was one of those, and it eventually stuck and today is widely accepted as the staple competitive format. It was the result of a lot of experimentation with class limits, unlocks and maps. It started out without enforced class limits, eventually the class limits got stricter (ex: demoman was limited to 1 per team) and the map pool more refined, . All this process culminated in the refined, fast-paced and interesting metagame we see today.
6v6 is played with six players on each team.
The common class format is:
This is also commonly referred to as the "Cookie Cutter" lineup, as it's the most versatile lineup which balances both mobility and damage output. Most classes have classlimits of 2, but Demoman and Medic always have classlimits of 1, with other classes such as heavy and engineer also being limited to 1 depending on the league ruleset.
Standard play involves a team of two Scouts, two Soldiers, one Demoman, and one Medic. The Demoman is usually the damage dealer and the most important class following the medic. The scouts play similar roles, but one is usually supposed to be more passive than the other. The soldiers play completely different roles, the Pocket Soldier protects the medic and spearheads pushes, while the Roaming Soldier helps the scouts and is usually the first player to initiate, or to create a window of opportunity for his team.
In some cases, a team swaps a class of the cookie cutter lineup for a utility class, in order to create an advantage for your team, to help in a defensive situation or even to break a stalemate. This is called offclassing. A scout or the roaming soldier generally switches, which allows classes such as Sniper, Spy, Engineer, Heavy, or Pyro to be utilized. In general, offclassing means trading off the mobility of the standard cookie cutter lineup for damage or positional advantage or even a suprise effect.
Snipers and Spies are often referred to as "pick" classes, as their different playstyles and ability to 1-hit kill any class are very valuable when attempting to gain an advantage. Pyro, Heavy and Engineer are often called "defense" classes as they trade the mobility of one of the scouts or soldier, for better area control and/or damage output.
Sniper is perhaps the most common offclass, and a scout is the most common offclasser.
In 6v6, a few different map types are played.
The most common is 5CP, or "5 Control Points", which requires a team to control all five control points in a map in order to win the round. Common 5CP maps include cp_granary, cp_badlands and cp_process.
King of the Hill
KOTH, or "King of the Hill", is a game mode where teams fight for control of one central control point in a race against the clock. The only widespread koth map currently used for 6v6 is koth_pro_viaduct.
A/D, or "Attack/Defend", are maps which use a stopwatch to time the attacking team in trying to capture all of the defending team's control points. The defending team then switches to the attacking team and tries to beat the opposing team's time. The only 2 common attack/defense maps are cp_gravelpit and cp_edifice.
Capture the Flag
CTF, or "Capture the Flag", are maps where a team has to capture the intel from the enemy base and take it to your own base to score a point. This gamemode is very rarely played in 6s and most leagues have dropped CTF maps of their map pool. One of the most famous 6v6 CTF maps is Turbine Pro.
Below is a list of organizations which currently have, or have had, 6v6 leagues or cups in the past .
- tf2.co.za (South Africa)
- Tf2Ladder (Xbox 360)
There are a few different places where you can play practice games of various game formats:
- TF2 Center - A successor to TF2Lobby catering to all levels of players, allowing players to join casual games of 4v4, 6v6, 9v9 (Highlander), Ultiduo and BBall. Offers optional mumble integration with hosted mumble servers. Available across all geographies, but particularly Europe, USA and Australia.
- ATF2.ORG - A way to access to different IRC channels on the Gamesurge network, like #tf2mix (for beginners), and #tf2.pug.na (for higher level players)
- TF2Pickup.net - Web-based 6v6/9v9/Ultiduo and BBall pickup site, catering to players with previous competitive experience. Considered to offer a higher level of competitive gaming than the others. Offers hosted game and mumble servers. Currently only available to European players, but promises global availability soon.
- TF2 Stadium - "The newest and sexiest open source website for TF2 lobbies." - TF2 Stadium Steam group description
- TF2 Pug.me - This is a higher level TF2 pug website where you will see anywhere from Top open to Top invite players.
-  - "PugChamp is a draft-organized pickup game (PUG) system, in which captains select players for their team from a pool of available players."- description from tf.tv thread