Put it is like this. If a person is all about doing damage and not worrying about defence odds are they are a damage dealer, most commonly of the glass cannon play style. If a person is more focused on helping others and making sure they can do their thing odds are they are a support, most commonly a healer or a buff/debuff style. And if someone is all about protecting others by absorbing damage and taunting enemies odds are they are a tank, likely one of the masochistic play styles.
In games without a class/role system, your class is your playing style. Or your class is your personality type, as different personalities will tend to be drawn to do different things and be better at some things over others.