In MOSS you can use audience to target content to different Audiences.  So when two different person access the same URL they will not see the same content.  They will see only the content related to their jobs or needs if the content owner used the audiences to target the content.  This is a cool feature that I should explain more in detail in a different article.  

Audiences can be used in document libraries.  You have to enable audience targeting in the document library settings.  Once this is done a new property will be added to the document where you can specify an audience for the document.   

If you log in with a user not member of an Audience, he will see all the documents even if you have targeted document to specific audiences.  

Audiences will not control the display of the document while you are in the document library or if you add the Web Part that displays the content of the Document Library.   

To display the content of a Document Library based on audiences, you need to use the Content Query Web part.   

In the properties of the Content Query Web Part you can specify to use the audiences to display the content (apply Audience Filtering).