The Concept of Presence
Presence represents the availability and willingness of a person to communicate. A simple example is the list of buddies you have in your instant messenger.
When they are online (meaning they are available and willing to communicate), your instant messenger gives you an indication to that effect.
Presence can also be enhanced to show where you are and how (since we are speaking about integrating many communication tools) you can be contacted.
For example, if a buddy is not in her office or in front of her computer, there is no way your instant messenger can have you contact her, unless other communication technologies are integrated, like pc-to-phone calling. With unified communications, you can know where your buddy is and how you can contact her… but of course if she wants to share these information.