What is the difference between a regular textbook and a "customized" textbook for a university?

Regular textbook ISBN:9780534402303



http://www.amazon.com/Precalculus-Circle鈥?/a>



Customized textbook Title: Precalculus with UNIT CIRCLE Trigonometry with Solution CD

ISBN: 9781111216931



Both textbooks are from the same author, who is COHEN.What is the difference between a regular textbook and a "customized" textbook for a university?
Usually there is not much difference between the books. Often you can use the regular textbook, but I would suggest contacting the instructor to make sure.