a particular revision, which is called the branch head. The
branch heads are stored in `$GIT_DIR/refs/heads/`.
+master::
+ The default branch. Whenever you create a git repository, a branch
+ named "master" is created, and becomes the active branch. In most
+ cases, this contains the local development.
+
+origin::
+ The default upstream branch. Most projects have one upstream
+ project which they track, and by default 'origin' is used for
+ that purpose. New updates from upstream will be fetched into
+ this branch; you should never commit to it yourself.
+
ref::
A 40-byte hex representation of a SHA1 pointing to a particular
object. These may be stored in `$GIT_DIR/refs/`.