Although with most booster packs you get a rare, around every eight booster packs, that rare is replaced with a mythic rare. Booster packs can be used for a variety of tournaments as well.
Some tournaments use an alternate rule called Rare Drafting. When the tournament using a rare draft is over, the rares, mythic rares, and foils (or anything predetermined to be valuable) are separated from the rest, and the winner gets to choose one of the cards, and the second player can pick one after that, and so on. In the edition Scars of Mirrodin, if you open a booster pack, the chance you will get a certain mythic rare or rare is 1 in 68, for uncommon it's 1 in 20, and for a common it is 1 in 11. Despite the large amount of commons, you get so many commons in one pack that the chance of getting a certain common is relatively high in a booster pack compared to the other rarities, because you don't get very many uncommons and rares.