Autocompleting/suggesting text field

I am looking too implement a text area which would as the user types each letter search a specific catagory of my database and return results based on first the relevance and second by a secondary column i will designate... All i really need is either a link too a script i can read through too get an understanding or a tutorial too run me through the basics.

Any help greatly appreciated...

I suppose an example would be handy

Ebay search, Yahoo, etc...