Web parts can be targeted to a specific audience in order to push information in front of a group of people for whom it’s relevant. Using this feature, you can place several web parts on a page, but when a user navigates to the page, they only see the web parts which are relevant to them.
An audience that you target the web parts to can be a SharePoint Group, Active Directory Security Group or Distribution List, or even a Global Audience (which by the way is setup through the Shared Services administration). The intended target audience is specified from the web part properties task pane in the “Advanced” section as shown below
Now here’s a very important point to keep in mind:
Targeting content to an audience is not the same as securing that content. Audience targeting and the SharePoint security mechanisms are two very different things and should not be confused with each other.