Prerelease tournaments (or prereleases) are events held one week before the release of an expansion set. Starting with Shards of Alara, Wizards of the Coast allowed local stores run the tournaments, which vastly broadened their attendance.
The tournament format is sealed deck. Players build decks with cards from the upcoming release set. Players build a deck with a minimum of 40 cards from booster packs received. The unused cards are considered their sideboard for the tournament.
Prerelease cards have been given out at Prereleases since Tempest.
Starting with Commander (2011)  certain sets featured achievement cards which were created after the succesfull introduction of achievements in Duels of the Planeswalkers. The achievement card sets certain tasks that are to be completed during the tournament. Examples of tasks are to play a certain card, to have a certain combination of cards on the battlefield or to make use of a newly intoduced mechanic. After the player has played all his matches, he shows his completed score card (on the back of the achievement card) to the tournament organizer to receive additional prizes. Sets with achievement card include: Avacyn Restored , Return to Ravnica , Gatecrash and Magic 2014.
Starting with Mirrodin Besieged prerelases featured gimmicks. These sometimes included prerelease packs, which are are boosters that are especially composed and packaged for prerelease events. On the July 18, 2013 it was announced by Wizards of the Coast that Magic 2014 was very likely the last prerelease to not have a gimmick.
Mirrodin Besieged had faction packs. Cards with the Mirran watermarks appeared only in Mirran boosters, and cards with Phyrexian watermarks appeared only in Phyrexian boosters. There was also one basic land in each booster. Foil cards would show up at about the same frequency as they did in regular boosters and didn't follow the rules about what booster they could show up in (both Mirran and Phyrexian foils could show up in either booster). 
For the Avacyn Restored Prerelease, stores were able to register for a limited number of Helvault Prerelease Kits along with the standard kits that come with every prerelease. When players at the special Helvault Events got achievements, they could each physically break a seal on their store's own (cardboard) Helvault. Once all the seals were broken, the store would open their Helvault and all the players would share the spoils within. 
Return to Ravnica
For the Return to Ravnica Prerelease, you could choose from five Guild Prerelease Packs. The options were Azorius, Izzet, Rakdos, Golgari, and Selesnya. Each Guild Prerelease Pack contained five Return to Ravnica booster packs, a guild-specific booster pack, a special alternate-art prerelease card unique to each guild, an achievement card, a Spindown life counter, a guild symbol sticker, and a letter from the guild's leader. 
Dragon's Maze Guild Prerelease Packs each contained a Guild booster from Return to Ravnica and one from Gatecrash.
A cardboard version of the Implicit Maze was used as a gimmick to mark players's progress in the tournament. Players could work collectively to advance their chosen guild through the Implicit Maze each round. There were prizes for each guild that got all the way through the maze, and special prizes for the first guild that did it. And when the maze was completed, all participating players would receive a premium (foil) basic land promo card. 
Theros features the first quest of the Hero's Path: Choose a Path. The Hero Prerelease Pack contains a spindown life counter, a Hero Card unique to a players' chosen path, one of five quiz activity cards, five Theros boosters, a special seeded booster featuring cards appropriate for that player's chosen path and one of the five playable Prerelease promo cards. While building decks and playing matches players can work together to find the answers to the questions about Theros on their activity cards. This social scavenger hunt offers players an opportunity to engross themselves in the lore of the new block and interact with their fellow heroes. Once a player has filled out the card, he or she can check the answer key to see if they completed it correctly. Once completed, they are finished with Quest #1 and ready to continue on the Hero's Path.
- ↑ http://www.wizards.com/mtg/images/daily/events/commander_achievement.pdf
- ↑ http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/arcana/957
- ↑ http://www.wizards.com/magic/magazine/article.aspx?x=mtg/daily/arcana/1070
- ↑ http://www.wizards.com/magic/magazine/article.aspx?x=mtg/daily/arcana/557
- ↑ http://www.wizards.com/magic/magazine/article.aspx?x=mtg/daily/arcana/933
- ↑ http://www.wizards.com/magic/tcg/productarticle.aspx?x=mtg/tcg/returntoravnica/productinfo
- ↑ https://www.wizards.com/magic/tcg/productarticle.aspx?x=mtg/tcg/gatecrash/productinfo
- ↑ http://www.wizards.com/magic/magazine/article.aspx?x=mtg/daily/arcana/1156