Career is the WAR term for character class, sometimes also known as profession. All player characters must choose a career upon character creation, after army selection. The army selected determines which career options are available, according to the table below. Careers are divided between four archetypes and six races.
Your goal is to sustain damage, lead the charge, and protect your allies. Tanks can be built to deal high amounts of damage, but this is generally left up to "DPS" classes.
Expect to die. Your party's healers should be focused on keeping you alive, especially in PvE combat, but accidents happen and they can be distracted. Don't take it too hard and ask for a resurrection.
In RvR combat, be aware of the softer players in the group. If you are one of two or more tanks in the group, make sure that one of you is guarding the casters, healers or otherwise. Your heavy armor and debuffing attacks will buy them time to escape or finish off the attacker, and they can more easily heal you if you are close by and protecting their lives.
Tanks are also the most fitting choice to lead a group into a fight. Heavy armor and a shield make you a target of other players quickly, and they'll be less likely to notice the smaller players sneaking up behind them for the kill. Unlike the other classes, you are capable of engaging many different targets at once and spreading the damage out evenly among them while the ranged fighters pick off stragglers.
Don't forget, you and the healer in the group are the most essential pieces. Tanks are just as useful as the other careers in Warhammer Online. However, this does not make your class alone more prevalent.