What does it mean to poke someone on Facebook?

I just got poked on Facebook?  Is there a support group for this?  I poked back and thought about just going on a poking rampage!  Just poking all my friends over and over.  Does it leave a bruise?  Is it like the game my daughter plays with her friends called "Touch" where she just, well, pokes them on the arm with her pointer finger?  Now THAT's a poke.  Can you tell it's a snow day and my kids are glued to the TV and I've got some time on my hands?