A much broader question than is readily apparent. This comes up every few months or so, and as such, I recommend you do a search on this site. I and others have answered this question in detail previously in older threads. I can leave you with a brief generality however: ....Letter checks are progressively more comprehensive as you go up and superscede one another-- that is, an "A" is included in a "B" and so forth. As you get into the heavier checks ("C" and up ) it is quite common to break them up into phases to perform portions thereof and minimize downtimes. Again, it's a tough question to answer easily due to the many variations in airline maintainance programs.