"Hey Dave, have you got a minute?"
It's easy to stick your head into the next cubicle to see if a colleague is busy, but what if they're on another floor, at another site or in another country? If you need to know where they are and what they're doing, you need presence.
We're all familiar with presence in one of its most basic forms - instant messaging applications. By just glancing at your buddy list you can see if your friends are online and if they're available to chat. Most applications don't just tell you if your friends are online, they also let your friends set their status for particular events such as "On the phone", "Be right back" or "In a meeting". That, in a nutshell, is presence.
Many organisations use consumer grade instant messaging, such as those offered by Yahoo!, Microsoft or Skype, as an ad-hoc presence system to reduce the chances of playing answering machine tag. The combination of presence and chat makes it easy to get a quick piece of information to or from someone without interrupting them too much. If you need to know if someone is attending a meeting, no need to pick up the phone for a 10 minute conversation if you can see they're at their desk and available to answer a quick "RU coming 2 the sales meeting?" instant message.
Of course business grade presence takes this to a whole new level. Presence is often rolled out in conjunction with Voice over IP, although it is possible to integrate presence features into a traditional phone system.
The most obvious benefit of this integration is automation - the system can detect when you're on the phone and alter your presence status accordingly. It can also alter your voicemail message - you've probably already come across messages that say "Dave is on the phone right now" rather than the old "Dave can't take your call right now". That's presence at work.