When a player is all-in that creates a pot (main) that anyone that is still in after that betting action can win.They must be in at the end of the hand also.
The all in player is only eligible for that pot.
If there is any more betting it creates a "side-pot" that will be determined seperately to the main pot and only the players with additional chips can win.
If another player ends up all in,it can create another side pot.
As for the last part of your question,you call if you have proper odds to call,or think you have the best hand.You fold if you think you are beat to keep it simple.
I hope this helps
Snow