Differences between Customer Portal and Partner Portal in Salesforce
Partner Portal is part of a companies Partner Channel Sales efforts. It is a portal focused more on Sales force automation and the efforts of those partners that sell your products to nurture the leads you pass to them, the leads they enter in themselves and the convert to Opportunity and subsequent sale.
Customer Portal on the other hand is more focused on the Service and Support of ones Customers.
The feature differences are that Partner Portal exposes the Leads and Opportunity objects whereas the Customer Portal does not. However, only the top tier of (Gold Partner licenses) Partner licensing exposes the Case object whereas this is standard in the Customer Portal.