Berserkers are the tank of the group. They focus all their energy on getting the enemy to attack them. They can withstand an insane amount of damage, while doing a fair bit too. Berserkers' main weapons are melee. He gets in the battle and doesn't leave until everything is dead, or he is.
Soldiers bring their gun with them everywhere they go. It gives them comfort to have it by their side always. Focusing on ranged combat, you stay away from the fight at hand. Try not to let an enemy get too close for comfort.
Medics are needed everywhere. If you're mortally hurt, a Medic is the only one that can help you. Sporting a medkit on hand, a Medic is the character everyone wants with them at all times. Their gun doesn't do much in the ways of damage, but it will get him through.
Runics have devoted their lives to the study of Quake's ancient rune magic. They venture into the Realms for knowledge of their enemy and hope that one day, they can give Quake a taste of it's own medicine. Your staff is only as good as the magic you use.