Audience feature in MOSS should not be mixed up with security features/trimming of data based on the user credentials and the resulting authorization. Instead, 'audience' lets you filter out undesirable data for the current user context. E.g.:- you would want to hide the sales data on the home page if the current user is not part of the sales team; this makes sure the marketing (or other) teams are not overloaded with unwanted information.
In MOSS, the idea is to setup a user/audience list such that webparts/lists can later show/hide the required information. This is supported out of the box.
Setting Up an Audience
From a shared service provider, you could add new audiences by creating rules using windows groups or distribution list or any of the property available against a user (name, address, department, manager etc).
To extend this audience definition, we could have custom property (say 'Day/Night Shift') added against a user from the Shared Services->User Profile settings. This makes sure that we can create rules based on this new custom property to show items specific to employee working in the day shift.
Consuming an Audience
Out of the box, sharepoint lets you apply audience-targeting for the following items
a.) WebParts - while designing the webpart, you could specify the target audience for this webpart. In this case, the webpart would be rendered only if the current-user is part of the target audience. In all other cases, the webpart would not be visible / rendered.
b.) Lists - While designing a list, you could specify that the list needs to have audience targeting available. In this case, you could use Content Query WebPart to filter out data
Consuming Audience programmatically
AudienceManager class - Acts as the entry point into the entire functionality of audience in MOSS. This class also implements the IRuntimeFilter interface to perform the targeting functionalities for webpart/lists.
For a custom user control to consume the Audience functionalities, one of the way is to setup an audience list for the custom control (a new property perhaps) such that at runtime, the control can use AudienceManager.IsCurrentUserInAudienceOf() method to check if the active user is part of the audience previously setup. Note that in this case, it’s up to the control developer to implement the required functionality of hiding/filtering data etc.