When one person pokes another on Facebook, it is simply a way of getting that person's attention. Poking can be aimed at friends or friends of friends.
If someone pokes a user, it opens up their Facebook profile to be seen by that person for 4 days. The recipient of the poke will receive a notification allowing them to see the person who poked them. No one else will know if one of their friends has been poked.
A person cannot be poked again by the same person unless the poke is removed, or if the user pokes back the friend who poked him first.