Madoka is a typical school girl until she meets a magical talking cat that wants to make a deal with her and turn her into a magical girl. As a magical girl Madoka would fight witches and stop them from cursing and hurting others. The magic cat has Madoka shadow one of his magical girls so that she can see what having powers is all about.
Now for those of you getting a distinct Sailor Moon vibe the resemblance stops there. Madoka's version is much, much darker. Think Sailor Moon with less humor, more blood, and a talking cat that definitely has a hidden agenda that has nothing to do with becoming the moon princess. In the first volume people get hurt and Madoka realizes that getting to have magical powers is not all sunshine and moonbeams. There is death and grief, and feeling all alone against the powers of evil. Also not all of the magical girls work together. There is some very serious rivalries going on between magical girls. The Sailor Scouts never threatened to kill each other.
Madoka Magika takes the normal magical girl story and turns it on its head. Take the serious tone of Claymore and put that into a story about a high school girl getting magic from a talking animal and you have Puella Magi Madoka Magica in a nutshell. For those who love the magical theme, but are sick of reading the same thing every time.
Give Puella Magi Madoka Magica a chance; just be warned that is not a lighthearted romp.