See the changes of a specific commit:
git diff <commit-sha> -p
Or,
git show --decorate <commit-sha> # See 'Author', 'Date' and 'diff'
See the diff of two commits:
git diff <commit1> <commit2>
See the file changes for a specific commit:
git show <commit>:<file>
See all the changes for a time duration (say, 1 day):
git whatchanged --since="1 day ago" -p
git whatchanged --since="1 day ago" -p <file> # See changes for a specific file only