A portal defines a specific user menu and access rights for its members. This menu can ben seen by portal members, anonymous users and any other user that have the access to technical features (e.g. the administrator). Also, each portal member is linked to a specific partner.
The module also associates user groups to the portal users (adding a group in the portal automatically adds it to the portal users, etc). That feature is very handy when used in combination with the module ‘share’.