imir, unlike blue-black in other sets, is known for trying to kill the opponent by running him or her out of cards. In Constructed, players have a lot of cards in their libraries, so that can be difficult. Fortunately, these days, Dimir has a number of creatures that can feed off the opponent's graveyard, so you can build around that element without actually needing to rely on it to win a game.
Today, I'm going to try to push milling as a support strategy for a creature deck. Jace's Phantasm, Wight of Precinct Six, and Consuming Aberration can all become extremely large creatures for their cost if you can be sure the opponent will have a large graveyard. Lazav, Dimir Mastermind will occasionally get to become an awesome creature, and even when he doesn't, he's still a 3/3 with hexproof against a deck without any good creatures.
Duskmantle Guildmage can let you do some work on your opponent's graveyard when you don't have anything else to do with your mana, but it's really there to turn Mind Sculpt into Searing Flesh, and if you can untap with Jace, Memory Adept with a live Duskmantle Guildmage in play, you can play a sixth land, activate Duskmantle Guildmage twice, and then use Jace's 0 ability to deal the full 20 damage in one turn.
The deck name is an inside joke from the first Ravnica Limited—Dimir draft decks could focus on attacking with evasive creatures or milling the opponent with cards like Vedalken Entrancer. Every now and then, a deck would have some of each, which you usually wanted to avoid, especially if the attacking creatures weren't good on defense, but there would be games where the only race was to see if you would win by damage or decking first—the classic "beat-mill."