The communicator team blog has a brief overview of the integration available between Outlook and Office Communicator and how this works.
Communicator Presence can be found throughout Outlook (Presence is the colored bubble that appears next to a person’s name).
In addition, conversation history (as well as missed-call history) is stored in Outlook to allow for a single location to search and review all prior communications. Finally, Communicator obtains calendar and out of office information from Outlook and/or Exchange and uses that information to affect your Presence state.
An interesting read, I've not given much thought to how this works because it just does.