An end user is an individual who has access to a Zoho Creator workspace. This can be anyone within an organization. They can access and add data on the application. The application owner can add users and control their permissions within the application.
A customer portal user can be anyone outside the organization who signs up using the portal, rather than signing up with a regular user license. This mostly includes customers, vendors, and business partners who you'll interact with on a regular basis. These users can log in to your dedicated portal, access applications shared with them, submit data, and view and edit their records.