Sounds like you are talking about a non-compete agreement: http://www.google.com/search?sourcei...ement+template
You could also get a non-disclosure agreement which would stop the company from revealing details of the project.
Honestly though, non-compete agreements are a waste of time generally speaking. You may stop the company you have employed from making a competing product but they would only be restricted for a short amount of time. Maybe a year. The fact is if you have a good idea it be a matter of weeks before people create cheap software which clones your sites functionality.
Ultimately an idea is only worth something if you can develop it. Until that day it isn't worth anything.