I would like to do a diff between two tags and committed changes between those two tags. Could you please tell me the command?

$ git diff tag1 tag2

or show log between them:

$ git log tag1..tag2

sometimes it may be convenient to see only the list of files that were changed:

$ git diff tag1 tag2 --stat

and then look at the differences for some particular file:

$ git diff tag1 tag2 -- some/file/name

A tag is only a reference to the latest commit 'on that tag', so that you are doing a diff on the commits between them.

(Make sure to do git pull --tags first)

Also, a good reference: https://git-scm.com/docs/git-diff