Partial Contact Role is an extension on the Opportunity Contact Role in SalesForce.  The main difference is that Partial Contact Roles allow for a Contact Role that is given only a partial Soft Credit on a Donation.  This is represented by a percentage.  The Partial Contact Role is ONLY used for partial allocations, while the Contact Role can be used for either.  This can cause both a Partial Contact Role and a Contact Role to be created for a partial Soft Credit.