Zerg Groups are as follows, Garm Brood is the lowest, Jorgumand Brood is the middle, and Kerrigan's Demolition Team is the best.  Each group is controlled by a cerabrate, and each cerabrate may appoint people to the different ranks, and may also give them special assignments. He may appoint as many as 5 people to each rank.

Cerabrate:  This is the leader of the group.  He is the representative to the congress and gives orders to every one of his underlings.

Queen:  This person is assigned to do the bidding of the cerabrate and one of them is chosen to be the cerabrate of a new brood.

Ultralisk:  This is a position where you are to take orders.  However, you are one step away from the highest ranks.

Guardian:  You are the highest ranking soldier so you are the person that has rank but still gets in on a lot of action.

Hydralisk:  This is a position where you don't have lots more privileges, but it is an easy rank to attain.

Zergling:  This is the lowest rank, but one of the best! You get to battle and battle till you can't anymore why the officers are up there in all the meetings.