Dark Lord of the Sith
The mystical and terrifying head of the Sith Order. He commands all the Sith when needed. He can fight strongly for himself and still pick up on mistakes taken  by other sith in Battle. There may be only one Dark Lord at a given time.

Sith Lords
Sith Lords are the High Council of the Sith. Although the Dark Lord's decision is final, these powerful Dark Jedi's, who are basically Sith Masters with enhanced Sith knowledge, they have some leverage in Sith Order decisions.  Only three Sith Lords are allowed at one time on the Council. They may be challenged by Sith Masters for their chair on that Council. Grounds for the challenge shall be issued by the Dark Lord.

Sith Masters
Jedi devoted to the Sith and have vast knowledge of the Sith's history and future. They note faults in the Sith, and will point them out to the Dark Lord. The Sith Masters will train there apprentice with great skill and honour. There is no limit to the number of Sith Masters.

Sith Apprentice
The right hand man of their master. A very powerful Jedi that can take command of the Sith in case their master is somehow destroyed. Of course, he will have to  be strong enough to fend off all competition from other apprentices vying for the position.

Sith Warriors
The main fighting force of the Sith. They do the bidding of the Dark Lord. They wield Lightsabers and rudementary control of Sith magic. Sith Warriors are respected by their peers and feared by their enemies.