Basically the format Game Maker saves your project as.  For example, when you save a Word document, it saves to a single .doc (or .docx) file.  Often development projects involve multiple files however (multiple files for different pieces of code, or separate files for images etc).  One reason for this is that it's easy to share.  Every time you update something, you don't have to upload a few MB to the server.  Also, this allows multiple people to work on stuff at the same time.

Game Maker saves everything into one file.  Good for a single developer I guess, since you won't have a whole heap of files floating around, however, makes it practically impossible to split parts up for multiple people.
A possible work around is if the file is text based (eg XML).  Version control systems like SVN can detect what part of a file you've changed, and "merge" your changes, rather than reupload a ~20MB file every time you need to update something.  However, the .gmd (Game Maker) file format is binary, thus this sort of merging doesn't exactly work well.

(This post was last modified: 25/09/2008 03:33 AM by ZiNgA BuRgA.)