Welcome to the world of Team Fortress 2 in a 6v6 setting. If you are here reading this, you have probably already played TF2 in a public game setting before and are looking for either a new game type or just a more challenging game experience. Most spectators of competitive TF2 are often confused as 'competitive' and 'goofy cartoon shooter' don't usually go together cohesively, but the game actually provides a lot of depth and balance in an organized game type. Teams almost always play CP (Control point) maps but occasionally CTF (Capture the flag) game type is utilized.
If this all seems confusing I will give a basic rundown of how 6v6 TF2 works. There are two teams comprised of 6 players (duh) that have a specific class layout. The general format for teams is to run two Scouts, two Soldiers, a Demoman, and a medic. The other classes are used, but they are based on specific situations which we will get into more depth later. Team Fortress 2 6v6 is a complex game setting but can be some of the most fun you'll ever have. Anyway, at the start of a game, both teams will ready themselves when necessary and wait for the rounds to start. At the beginning of each round each class has a specific 'rollout' which is necessary to get to the middle cap point the quickest. Once both teams reach mid, they battle out the initial mid fight which is considered the most important part of the round since it usually determines who gains the initial ground and is on the aggressive side of moving forward. From there on out, the game basically dissolves into a tug of war of sorts as to capping each other's five points.
SCOUT
This is the most mobile class in Team Fortress 2 6v6 as he has starts with a 133% movement speed and has a double jump capability. He however has the lowest HP, which makes him fragile and reliant on dodging and avoiding dangerous fights. In Team Fortress 2 6v6 games, he is mostly used for capping points quickly and working flank attacks. If we were to compare the scout to roles in other games he would be considered a DPS character. This class utilizes a hitscan weapon, so the skill ceiling is entirely aim based, but movement plays a large part in learning as well.
SOLDIER
The soldier is the slowest class in Team Fortress 2 6v6, with only 80% movement speed but he has many tools which help his mobility and maneuverability. He can use his Rocket Launcher to do rocket jumps which increase movement speed greatly at the cost of a small amount of health. He also has an 'Equalizer' melee weapon which increases his run speed depending on how low his health has.
In TF2 6v6 games, each of the two soldiers plays a different role; there is a 'pocket' soldier and a 'roaming' soldier. The pocket soldier is the player that stays with the medic the entirety of the game and main concern is keeping the medic alive and controlling the pace of their team. Basically he has most of the responsibility regarding almost everything. The roaming soldier plays off almost no heals, and relies on health packs and may also utilize gunboats (The secondary weapon which reduces rocket jumping damage greatly). His main role is to release pressure off the 'combo' (The pocket soldier and medic) and create a lot of havoc and damage in middle fights. The role is not clearly defined at all points in a game, but other purposes they may have is helping the scouts on the flank (opposite side the combo fights on).
DEMOMAN
Demoman is considered to be the most damaging and almost the most mobile (93% movement speed) class in Team Fortress 2 6v6. He also relies on damaging himself to move quickly using his sticky launcher; the air control is a lot easier and smooth. He is limited to only one in a game simply because of his sticky launcher. It is incredibly powerful. It doesn't require precise aim, the damage doesn't falloff at any distance, you can trap off entire entrances and it causes burst damage from in the air or on the ground.
It's not all roses and candy for him however, as he really doesn't have many options in close fights, and is very vulnerable to any classes that can get into this range when he is fighting. Due to this, he relies greatly on the help of his team and positioning in fights. In Team Fortress 2 6v6 he is limited to one per team, due to his incredible strength.
MEDIC
Considered the most vital class on any team, this is how you get the heals! This class has a 107% movement speed but only has basic running for mobility. He heavily relies on his team for survivability but it also goes vice-versa which makes this class very interesting. The main feature of this class in Team Fortress 2 6v6 is the ubercharge which grants 10 second of invincibility and enables most teams to begin their push. The percentage of this charge (0-100%) also creates a big 'meta-game' of when an opposing team will test their luck and pop their uber first. The medic can also utilize the Kritzkreig, which is like an ubercharge but instead it grants five seconds of critical attacks, which is also useful for killing a team quickly or making an opposing medic 'drop' uber. The most popular kritz targets are Soldiers and Demomen.