A light weight tag that was created using git tag NAME is a simple file saved as .git/refs/tags/NAME that contains the SHA1 of the current commit when the tag was created.

An annotated tag created using git tag -a NAME is a simple file saved as .git/refs/tags/NAME holding the SHA1 of a newly created object of type "tag" that was saved in .git/objects.

$ git cat-file -p SHA1

$ git push --tags