There are 3 types of  Salesforce.com Portals:

• Customer Portal
• Partner Portal
• Self  Service Portal

The feature differences are that Partner Portal exposes the Leads and Opportunity objects whereas the Customer Portal does not.

Customer Portal is to support your customers.

Partner Portal is to support your Partners. Partner users are Salesforce users with limited capabilities. They are associated with a particular partner account, have limited access to your organization’s data, and log in via a partner portal. But when compared, Partner portal is costlier than Customer Portal.

Self-Service provides an online support channel for your customers – allowing them to resolve their inquiries without contacting a customer service representative.

A Salesforce.com Customer Portal is similar to a Self-Service portal in that it provides an online support channel for your customers—allowing them to resolve their inquiries without contacting a customer service representative.