Differences with growl module?

Sure. The growl project is aimed at developers who want to view watchdog messages in real time in Growl. From the module page: "A convenience utility for Mac-based Drupal developers, this module pipes watchdog messages to the Growl notification framework."
It also relies on having a command line tool installed called 'growlnotify'.

My module is different in that its purpose is to enable messages to be sent to arbitrary Growl listeners on a network. It doesn't require any command line utility to be installed as it uses PHP to form the packets sent to Growl. It also contains a module that enables the messaging framework and the notifications module to send out messages. This becomes very useful for Intranet systems like Open Atrium as they can notify users through Growl rather than email if they choose.

Hope thats clear enough!