Contact Hours are hours earned before becoming certified. Contact Hours can only be earned as traditional course time that focuses on project management.
PDUs are earned after one has become certified and there are many ways to earn PDUs.
There are 5 Categories which PDUs fall under.
Category 1: Formal Academic Education - courses taken in a degree granting program related to project management.
Category 2: Professional Activities - authoring articles in Journals, speaking or teaching PM related courses, developing a course related to PM, practicing project management and self directed learning activities.
Category 3: PMI REP - courses taken through registered education providers and chapter events such as meetings.
Category 4: Other Provider - courses or seminars taken through organizations not registered with PMI.
Category 5: Volunteer Services - services provided to your local chapter, other PM related organizations or charitable organizations.
PDU Calculations and documentation requirements can be found in the Handbook of your respective certification and does provide a more detailed description of each activity.
If you have any questions, you can also write directly to PMI Customer Care at firstname.lastname@example.org. Their response time is a bit slow though (about 3 business days).