ASDF is Another System Definition Facility: a tool for specifying how systems of Common Lisp software are comprised of components (sub-systems and files), and how to operate on these components in the right order so that they can be compiled, loaded, tested, etc.
ASDF presents three faces: one for users of Common Lisp software who want to reuse other people's code, one for writers of Common Lisp software who want to specify how to build their systems, one for implementers of Common Lisp extensions who want to extend the build system. See Loading a system, to learn how to use ASDF to load a system. See Defining systems with defsystem, to learn how to define a system of your own. See The object model of ASDF, for a description of the ASDF internals and how to extend ASDF.
Nota Bene: We are preparing for a release of ASDF 2, which will have version 2.000 and later. Current releases, in the 1.700 series and beyond, should be considered as release candidates. We're still working on polishing the code and documentation. “What has changed between ASDF 1 and ASDF 2?”.