A module is a complete set of scripts and typically given a code name. The current module is dubbed "LE" which doesn't really stand for anything in particular, though you could consider it the Limited Edition if you like.
Once the code in a module reaches a satisfactory level of performance and bug reports are few and far between, a new version of the ACP is released. Releases are available for download on the ACP SourceForge project pages under the Files section. They are listed by module (XL, LE, etc) and release name (XL1, XL2, etc). If in doubt as to which module or release you should download, always check the timestamps on the files and download the most recently dated one.