Trapping is a key element in quality four-color printing. It also tends to confuse and befuddle many professionals because, not only does each software program require a different process for trapping, but each piece of art or text may require individual attention during the process.
Digital Prepress for Comic Books deals with this subject throughout the entire book, and contains a 15 page appendix devoted to the topic. If you are interested in a more complete resource on the subject, I recommend the book The Complete Guide to Trapping by Brian P. Lawler from Hayden Books (ISBN#1-56830-098-0). Below are screen shots of two pages from the book covering general trapping guidelines.