Google Apps Scripts allow you to:
- Automate repetitive business processes
- Integrate 3rd party services within Google Apps
- Create custom spreadsheet functions
- Build and collect user inputs
With Google Apps Scripts you can interact with Google Base, Calendar, Contacts, Document, Finance, Language, Mail, Maps, Spreadsheets, and now Google Sites.
In addition to accessing Google Products they provide many tools for working with UI, SOAP, XML and other helpful utilities.
Google Sites moves beyond a simple wiki with the addition of Google Apps Scripting. It now is more of an application platform allowing integration with other Google products and 3rd party services.