W:ET has two teams: Axis and Allies. Each team has Soldiers, Medics, Engineers, Covert Ops, and Field Ops classes. Each class has different abilities and weapons. The game combines Quake-like all-out fragging with strategy and changing objectives. You can be Rambo or you can be more stealthy and sneaky.
Each map has several key objectives. Typically, if one team's objective is to build or protect something, the other team's objective is to destroy it.
If you're new, the best the class to be to learn the game is to be an engineer, as engineers are absolutely required to do anything in the game. The simplest map is an allied beach landing where the allies must build a ramp to get up over the beach cliff, capture a remote bunker (a spawn point), infiltrate a Nazi base, and blow up massive anti-ship guns.
To do this, Engineers must build the ramp using their Pliers (press 5 on keyboard), and blow up the guns by dropping dynamite (6) and arming it using the pliers. Axis can blow up the ramp Engineers build by using dynamite as well. Both teams can defuse dynamite by using pliers.
Other objectives are:
For completing objectives and doing well in the game, you are rewarded with "experience points." The more points you get in various categories, the more your skills in that category are upgraded. Such as increased health capacity, faster reload time for weapons, or being faster at engineering something.
When you get "critically injured", you are immobilized and must press Space to jump back into the queue for redeployment at a spawn point. Or, you can wait for a medic to come by and revive you if there are any medics. Press M to call for a medic.