tar on a directory mydir will archive hidden files and hidden subdirectories, but tar from within mydir with a * wildcard will no
You can compress all the files / folders in your current directory (including .hidden) by using:
tar -zcvf compressed.tgz `ls -A -1`
The last argument are the folders you want to compress. If you pass it ls -A -1 , you are passing it all folders in your current directory but . and .. .
When it comes to subdirectories, .hidden files are already included in the compression by default.