posted
As I understand it, tar technically isn't a compression program, but an archive program. It was historically used for tape backups. gzip is the actual compression program. Files wit .tar.gz or .tgz extensions have first been run through tar to archive them, and then run throug gzip to compress them.
gzip is indeed available for Windows, and I'm pretty sure most versions can tar and untar files as well as gzip and gzip -u them.
I'm not sure if you'd get better compression rates with .tgz or with plain old .zip. If the file is being distributed to Windows folks, I'd stick with .zip just for ease of use. .tgz is more common with flavors of unix.