Examples 1 - 5 Scripting in spreadsheets
Make a copy of the scripting examples spreadsheet from here.
Example 6 Scripting and Google Sites
Scripting on a Google Site
Creating you own user interface in a spreadsheet - step by step
You also can create a user interface on a site page have the data submitted to a spreadsheet.
The following example does not work but it does show how you can create a customized user interface - an upgrade to a form.
Pats In Progress User Interface on a Site
Guide for Apps script: http://code.google.com/googleapps/appsscript/guide.html
Examples of Apps Script: http://code.google.com/googleapps/appsscript/articles.html
Google Script Expert Site with examples especially for UI services: http://sites.google.com/site/scriptsexamples/home