From the introduction:
Welcome to a world of magical girls, Jumper! In this world, evil monsters known as Wraiths stalk the world at night, unseen by ordinary people. They are confronted and defeated by Magical Girls, who make contracts with a mysterious being named Kyubey to fight these beings and sustain themselves on the energy they provide. In return, Kyubey grants them a single wish, and even gives them eternal life so long as they can keep up with their energy reserves. Should they fail to do so, or fall in despair, they are said to vanish from existence. And now, you’re one of these Magical Girls, Jumper! Or... Magical Boy, of course. Bending the rules a bit for you is fairly easy to do. Eh? Bending the rules? Jumper? What are you talking about? How did you...
Who’s Madoka? Don’t worry about it.