Charmed is about Prue, Phoebe and Piper Halliwell.
They are the Charmed Ones the most powerfull witches ever. Together they have the power of three. With the help of Piper's husband Leo, who also is a guardian angel called a whitelighter, and Phoebe's husband Cole, who also has a demon form called Beltazor, they protect the innocents against demons, warlocks and other things of darkness. After Prue was killed by a demon, Phoebe and Piper found out they had a half-sister called Paige. With her help the Power of three was complete again.