Dredge

Dredge is the mechanic atributed to the Golgari in the Ravnica Block. It is a mechanic of recursive nature, where the owner of a card in a graveyard with said ability can choose to skip his draw and put an amount of cards in his library into his graveyard (number which varies from card to card). It only appears on green and/or black cards. Quoting from the Comprehensive Rules:

502.47. Dredge

502.47a Dredge is a static ability that functions only while the card with dredge is in a player's graveyard. "Dredge N" means "As long as you have at least N cards in your library, if you would draw a card, you may instead put N cards from the top of your library into your graveyard and return this card from your graveyard to your hand."

502.47b A player with fewer cards in his or her library than the number required by a dredge ability can't put any of them into his or her graveyard this way.

One of the most popular uses of dredge is to use it to quickly fill your graveyard. A famous deck, Ichorid, originated from this concept. To those purposes, cards with high dredge numbers are used (mainly Golgari Grave-Troll and Stinkweed Imp). Another class of decks using this mechanic that debuted in the tournament scene are Life from the Loam decks, which use the card Life from the Loam to gain card advantage by continuously recycling lands wih cycling, as well as other utility lands like Wasteland and Cephalid Coliseum.