What is the difference between Instant Messenter and Ray Instand Messenger (And while we're at at - what is Presence really?)

In the admin control panel, it gives you the option ot acitvate both Instant Messenger and Ray Instant Messenger. This prlobably sounds silly - but what is the difference and what one is where?

And, is Presence just a "who's on line feature" ? or is it another messenger product too? I must admit that Presence kinda looks cool, whatever it is.