yah it's a javascript framework.. popular ones are:
- mootools
- jQuery
- prototype / scriptaculous
- YUI - yahoo user interface
makes things all AJAXie and stuff

they are essentially very similar in nature (they are ALL .js, so that makes sense) and offer pretty much all you'd want, so it comes down to a preference, imho..
i started out loving scriptaculous and have since switched to jQuery, mainly because it's the defacto for Wordpress and i'm now very familiar with its quirks and features.