CPD specifically refers to professional development for speech/language pathologists and audiologists. In these professions you work with students who have speech or hearing problems. The development courses keep in line with the current best methods of improving communication. As new programs and ideas develop, courses encompass the changes to help you provide the best information to both your students and the parents. CPD units are required in order to keep you license current as a speech pathologist, audiologist or both.
PDU has been designed for all educators. While courses may be specific to certain areas of education, overall the idea remains to help inspire you to keep your material current. In order to stay current with your teaching license you must take a certain number of PDUs per renewal period. You can choose courses that fit with your subject matter or age group in order to make the development courses worthwhile and pertinent.
Some organizations allow crossover courses. Because these are continuing education courses, learning about new forms of the education process can be useful to speech pathologists and learning about speech deficits can be beneficial to teachers. Your state will have a list of approved courses that count towards your specific licensure. If you hold both licenses, you might be able to use some of the credits for each renewal.
Every state sets its own standard for how many credits are required in order to keep your license current. States also decide what you must do if you let your license lapse or are going "inactive" for any period of time. A few exceptions are offered for extreme circumstances, but in general it is advisable to stay on top of your professional development needs. Check with your employer to see if courses will be offered through the organization. It is in the employer's best interest to keep you current, so often the employer commissions classes to help fulfill the credit requirements for many employees at once.