ASDF is Another System Definition Facility: a tool for specifying how systems of Common Lisp software are made up 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. If you are new to ASDF, see the quick start guide.
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, and one for implementers of Common Lisp extensions who want to extend the build system. For more specifics, 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.
Note that ASDF is not a tool for library and system installation; it plays a role like make or ant, not like a package manager. In particular, ASDF should not to be confused with ASDF-Install, which attempts to find and download ASDF systems for you. Despite the name, 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 (http://www.quicklisp.org) instead, a Common Lisp package manager which works well 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 (http://common-lisp.net/project/clbuild/). We recommend ~/common-lisp/ as a place into which to install Common Lisp software; starting with ASDF 3.1.2, it is included in the default source-registry configuration.