mail server problem

hi,

Can someone please explain the difference between an open relay 
and a closed relay ?

How can i determine whether my mail server is an open relay or a 
closed relay ? And what steps are required if i want to make my 
mail server a closed relay ?

My mail server is being listed automatically on some spam blocking 
sites and i am not able to send/receive mails from some of my 
contacts.

thanks,
swanand