What is a White lighter?

A white lighter is a messenger of sorts. They are protectors and guides for good witches and future white lighters. White lighters are lead by a group of elders known as the Founders. The Founders make the decisions about who the white lighters protect and what the "job" entails.

Leo was sent to watch over the Charmed Ones when
they first got their powers though they weren't supposed
to find out his true identity.

 

In "Secrets and Guys" someone finds out about Leo's secret and, that someone is Phoebe Halliwell. Phoebe walks in on Leo while replacing light bulbs. She is stunned, to say the least, to see Leo floating in mid air and holding a lit bulb in his hand. After falling down, Leo begs Pheebs to let him explain. Once she is convinced he isn't a demon, Leo explains who he really is and what he's doing there. Leo also explains how white lighters aren't allowed to fall in love with witches but he has fallen in love with Piper.

 

Leo's Powers:

Leo has several powers he uses to protect his charges. First and for most, Leo has the ability to heal. All he has to do is lay his hands over what needs to be healed and presto!

He also has the ability to orb into and out of places. In other words, he can appear and disappear whenever he pleases. 

Leo also has the power to levitate.

Other powers we've seen in recent episodes are, the ability to give a subliminal message ("The Devils Music") and erase memory. Hopefully we will be seeing more of his powers in future episodes.