Customer Portal is an externally facing site that can be set up in an org to provide customers of an org limited access to data and functionality by logging in to the Customer Portal.
Questions related to the Salesforce Customer Portal should use this tag.
The specific data they can access can be configured, but is generally related to their account such as cases and also to other general data. Customer Portal is marketed as being a part of Salesforce's Service Cloud offering. Note that an org can configure more than one Customer Portal.
Closely related to the Customer Portal is the Self-Service Portal. Starting with Spring '12 the Self-Service Portal is no longer available for new organizations.
For more information see the following.