Every branch is simply a named pointer to a commit. A special unique pointer for every repository is the HEAD which points to the commit the working tree state currently corresponds to. The HEAD cannot only point to a commit, but also to a local branch, which itself points to a commit. Committing changes will create a new commit on top of the commit or local branch the HEAD is pointing to. If the HEAD points to a local branch, the branch pointer will be moved forward to the new commit; thus the HEAD will also indirectly point to the new commit. If the HEAD points to a commit, the HEAD itself is moved forward to the new commit.
原文：http://www.cnblogs.com/zhoug2020/p/4306953.html