Note: the original website address in here
Technical notes
The contents of files are not actually stored in the index (.git/index) or in commit objects. Rather, each file is stored in the object database (.git/objects) as a blob, identified by its SHA-1 hash. The index file lists the filenames along with the identifier of the associated blob, as well as some other data. For commits, there is an additional data type, a tree, also identified by its hash. Trees correspond to directories in the working directory, and contain a list of trees and blobs corresponding to each filename within that directory. Each commit stores the identifier of its top-level tree, which in turn contains all of the blobs and other trees associated with that commit.
If you make a commit using a detached HEAD, the last commit really is referenced by something: the reflog for HEAD. However, this will expire after a while, so the commit will eventually be garbage collected, similar to commits discarded with git commit --amend or git rebase.
