The official explanation of presence was "Presence is a real-time indicator of a person’s willingness and availability to communicate".
Ya, that explanation is as clear as mud. Let's take a look at some real life examples.
If you're reading this blog, you're unlikely to be unaware of "Instant Messaging" (or IM). Just to name some of the IM software: MSN Messenger, Yahoo Messenger, AOL Messenger, ICQ, IBM Sametime.
Obviously, the major purpose of IM software is to send instant message. Yet another important information they provide is buddy status - 'Available', 'Away', 'Out to lunch', 'In a meeting', 'Do not disturb', etc. Those status are 'presence' info, and play a more and more important role in our life.
With the adoption of IP phones, more and more features were built on it. In the old world, you call somebody and got a busy signal. You know he's on the phone. In the new world, you could see if he's on the phone or not, before you actually call him. This is called 'phone presence'.
Let say, you'd like to ask your buddy out for lunch. With presence tool, you know he's on the phone. You would use other method to communicate, such as instant message, email, etc. With appropriate tools, you can even send the instant message to his IP phone.
If you'd like, you may also have the presence software publish your calendar status. So your buddy would see your status as 'In a meeting' when you have a meeting going on in the calendar.