Prevent employees from using anything else but my proprietary application
You will likely want to look at the Parental Controls (System Preferences). While it won't prevent them from moving around in OS X, it will prevent them from running other applications that you have not explicitly authorized.



A detailed view rundown can be found here: http://www.dummies.com/how-to/content/how-to-limit-the-capabilities-of-a-user-in-mac-os-.html