The Protoss have three groups that are as follows, the Union of Adun which is the lowest ranking, the Executors Warriors which are the middle, and the Templar Elite which are few but their courage and skill is unsurpassed.  Each of these groups has a praetor, who in turn is the groups representative to the congress. 

Praetor:  This is the leader of the group and is the representative to the congress.  He assigns people to the ranks (he may assign as many as 5 to a rank) and gives orders.

Arbiter:  This person gives the orders to the soldiers and new group leaders are chosen from these people.

Archon:  This person is supposed to take orders, but it is a good rank to be as you are only 1 rank from a high position.

Scout:  This is the highest ranking soldier and it is a good rank because you get to be in battle but you are still at a good rank.

Dragoon:  This is a rank that you will primarily take orders. However, you can get to this rank rather quickly. 

Zealot:  This is the lowest rank, but one of the best!  You get to battle all the time while all the officers are in meetings.