git-annex uses a key-value abstraction layer to allow file contents to be
stored in different ways. In theory, any key-value storage system could be
used to store file contents.

When a file is annexed, a key is generated from its content and/or metadata.
The file checked into git symlinks to the key. This key can later be used
to retrieve the file's content (its value).

Multiple pluggable backends are supported, and more than one can be used
to store different files' contents in a given repository.

* `WORM` ("Write Once, Read Many") This backend stores the file's content
  only in `.git/annex/`, and assumes that any file with the same basename,
  size, and modification time has the same content. So with this backend,
  files can be moved around, but should never be added to or changed.
  This is the default, and the least expensive backend.
* `SHA1` -- This backend stores the file's content in
  `.git/annex/`, with a name based on its sha1 checksum. This backend allows
  modifications of files to be tracked. Its need to generate checksums
  can make it slower for large files.
* `URL` -- This backend downloads the file's content from an external URL.