Online presence enables individuals who have access to your site to see who is online and to send instant messages to them
Online presence gives individuals the ability to identify who is online and available to communicate with them at any given moment. Enabling online presence (and installing the required software) adds an online status indicator next to an individual's name wherever their name appears in a site collection. The online status indicator shows whether the individual is offline or is online and available to respond to queries via an instant messaging client. When an individual is online, you can click the online status indicator to send an instant message. This direct access to knowledgeable sources can help team members work more effectively and efficiently.
Wherever a member name appears in a site, the online status indicator menu is available. The online status indicator is integrated with the Active Directory directory service, Microsoft Exchange Server, and MSN Messenger to provide information, such as office location and free/busy status, so it is a useful tool for scheduling meetings and sending e-mail even when an individual is offline.