PDU is an abbreviation of Professional Development Unit.
1 Hour of learning/training is considered as 1 Contact Hour by PMI and is provided by the Training Providers. To be able to go for PMP Exam, one would need 35 Contact Hours.
Yes, PDU are little different than contact hours as considered by PMI. Before exam is Contact Hours, after exam is called PDU.
Every PMI credential holder is supposed to maintain the credential by constantly upgrading themselves by relearning/retraining on relevant Management topics in defined time frame by PMI. Every 1 hour spent on upgrading is referred to as PDU by PMI. To be able to maintain PMI credentials, one has to acquire relevant amount of PDU’s. For example, PMP credential will require you to maintain 60 PDUs in 3 years time frame after passing the exam.