Dave,

A blind is a forced bet that a player must make to get the action started. It is called a blind because he is betting before he sees his cards. This is instead of an ante. In hold'em and Omaha there are typically one or two blinds to the left of the dealer. Where two blinds are used the player immediately ot the left of the dealer typically posts a portion of the bet and is called the small blind. The player to his left must place a full bet and is called the big blind. The player to his left must then either call, raise or fold.

I have seen the absent button used where a player temporarily got up from the table and the dealer uses the button to signify that no cards are to be dealt to that player.

I haven't seen the present or player returning buttons before. I could speculate on their use, but I'm sure someone else knows for sure.