How Sharepoint 2003 license works

Hi All,

Can you please let me know how the Sharepoint portal server 2003
license works.
Say for example if I have 500 user licenses for accessing the
Sharepoint sites and if my vendor says that my license has been
exceeded then how can i track my active user count who have access to
Sharepoint sites.

On which basis my vendor will be tracking my license usage

Thanks