White magic is like prayer but without the middleman (God, Jesus, Allah, whoever).
It's the direct sending of one's beneficial will into the ether using a somewhat formalized ritual (casting a circle of protection, calling on the "power" of the four corners -- North, East, South, and West, and other rituals that depend on the particular form of Wiccan belief).
Margot Adler's "Drawing Down the Moon" thoroughly explains what Wicca and white magic is all about, if you're interested.
Do I believe in it? No.