Praetor is a creature type.
Praetors are powerful beings that live in New Phyrexia. Currently, there are five Praetors that live on New Phyrexia, which used to be Mirrodin. All are extremely powerful mythic rares. They are a cycle, which means there is one of each color. Each one is the brutal tyrant of its own faction. Elesh Norn, Grand Cenobite is a white Praetor. As you can see, Praetors are expensive but all will probably win the game if they can survive a turn or two. All the Praetors except Urabrask have one keyword ability; Elesh Norn has vigilance, and Vorinclex has trample, Jin-Gitaxias has flash, and Sheoldred has swampwalk, while Urabrask gives himself haste through his own ability. All have a converted mana cost higher than 4, and all except Urabask have a converted mana cost above 6. The most expensive Praetor is Jin-Gitaxias, Core Augur, with 10. Although the Praetors are mentioned in Flavor Text in Scars of Mirrodin and Mirrodin Besieged, the do not show up as cards until New Phyrexia. Praetor is a creature subtype, and one only seen on two cards in prior sets (Ebon Praetor and Sanguine Praetor).
During development of New Phyrexia, the Praetors were actually created because of the cycle of the Exarchs. Exarchs are cards that are creatures and when they come into play, they give you a choice of doing something bad to your opponent or good to you. For example, Inquisitionor Exarch says that when it comes into play, you may have target opponent lose 2 life or you gain 2 life. Not all the Exarchs do that sort of thing, but they all do something either bad to opponents or good for you. This led to the cycle of the Praetors.
Praetors are in charge of New Phyrexia and work hard in completing the Grand Work, which is making New Phyrexia a perfect place. Phyrexians do that. They strive to make things perfect. (Check out the Flavor Text of Phyrexian Obliterator to see. (Phyrexian Obliterator is in New Phyrexia.).) The Praetors each design Phyrexia to perfection.
Praetors, like all Phyrexians, are certainly scary and dangerous creatures. They do not let anyone get in their way of the Grand Work. They do not get involved with each other very much either, and most of the Praetors consider Urabask to be weaker than the other Praetors (which he probably is.)
When New Phyrexia becomes ready for drafting, it seems clear that Magic: The Gathering players will all be ready to get their hands on the Praetors, which are some of the best cards in Magic the Gathering.
The Praetors - What they do
Elesh Norn, Grand Cenobite
Elesh Norn is a 7 (converted mana cost) for a 4/7. Take a look at its abilities ... it has vigilance, and it gives each other creature you control +2/+2 - and it gives each of your opponent's creatures -2/-2, a whole lot more powerful than Infest. While the 4/7 won't win you the game alone (most likely), the -2/-2 and +2/+2 might.
Jin Gitaxias, Core Augur
This card is a bomb - in a different way than most people think of as "bombs," like Hellkite Overlord. This card will tear apart your opponent's hand - and because it has flash (gah!) you can cast it during your opponent's end step while they are tapped out and have them discard every single card. Then you yourself draw seven cards during your end step. Be sure to keep a Spellbook or a Venser's Journal down, as with all those cards you will draw, you'll probably have to discard a whole bunch with the seven card restrictment. And also, be sure you don't drain your library ... keeping this card down too long can make you lose the game if you don't play fast.
And also, this card is a threat to Jace, the Mind Sculptor. Eventually, shuffling your hand into your library (while exiling the rest of your library) won't be as much of a deal anymore.
Urabrask the Hidden
Urabrask is a great card for fast decks. He gives all your creatures haste. And in fact, he will do the exact opposite to your opponent's creatures. They all come into play tapped! With all that haste, your opponents, with their tapped blockers, won't have much defense to yours. However, he's a 4/4 - the weakest of the Praetors, and also, probably, the weakest in abilities as well. In a fast deck, though, Urabrask will speed you up and slow down your opponents, and he's the cheapest Praetor. He's not a total bomb though, and you'll most likely need more power to use Urabrask to his best.
Sheoldred, Whispering One
While Sheoldred is a 6/6 for 7 mana, that's not the best of her. She has swampwalk (decks with only black, watch out) .. okay, well, keep reading. At the beginning of your upkeep, return a creature card from your graveyard to the battlefield. You can make ridiculous combos with this card; bring back all your big creatures and slam them all back onto the battlefield. And it's the battlefield, not your hand. No casting cost or anything. You can return Jin-Gitaxias to the battlefield and eliminate all that 10 mana you have to pay. And that isn't it, as if that's not enough. Sheoldred also gets rid of your opponent's creatures as well. At the beginning of their upkeep, one of your opponent's creatures will go bye-bye (if they have any left). Now this Praetor is what you would call a "bomb." All she needs is a few turns to survive and the game will be yours.
Vorinclex, Voice of Hunger
Finally, the green Praetor comes. Vorinclex is a hefty 7/6 with trample for 8 mana, which already is a nice powerful creature to get down. Vorinclex also, really, doubles the amount of mana you own with its next ability. Ever heard of Extraplanar Lens, Keeper of Progenitus, or Vernal Bloom? Vorinclex is far less specific when it comes to land colors. Any colored land you control can be tapped to add one more of the same color to your mana pool. That means that not only can it be any mana, but it only works for you. And Vorinclex, like Urabrask, slows down your opponents - in a different way. Whenever your opponents tap a land for mana, it stays tapped durn their next untap step.