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 have released ASDF 2.000 on May 31st 2010, and ASDF 3.0.0 on May 15th 2013. Releases of ASDF 2 and later have since then been included in all actively maintained CL implementations that used to bundle ASDF 1, plus some implementations that didn't use to, and has been made to work with all actively used CL implementations and a few more. See “What has changed between ASDF 1 and ASDF 2?”. Furthermore, it is possible to upgrade from ASDF 1 to ASDF 2 or ASDF 3 on the fly (though we recommend instead upgrading your implementation or its ASDF module). For this reason, we have stopped supporting ASDF 1 and ASDF 2. If you are using ASDF 1 or ASDF 2 and are experiencing any kind of issues or limitations, we recommend you upgrade to ASDF 3 — and we explain how to do that. See Loading ASDF. (In the context of compatibility requirements, ASDF 2.27, released on Feb 1st 2013, and further 2.x releases up to 2.33, count as pre-releases of ASDF 3, and define the :asdf3 feature; still, please use the latest release).
Also note that ASDF is not to be confused with ASDF-Install. ASDF-Install is not part of ASDF, but a separate piece of software. ASDF-Install is also unmaintained and obsolete. We recommend you use Quicklisp instead, which works great and is being actively maintained. If you want to download software from version control instead of tarballs, so you may more easily modify it, we recommend clbuild.