A sortable list is simply a list of items that can be dragged and dropped to alter the order of those elements.
There are two sortable lists available in Wordpress, one from the JQuery framework and one from the Scriptaculous framework. For a sortable list you will need a list, so here is a simple one.
Note: This code has been written with WordPress 2.8 in mind. It won't work on versions prior to 2.6 and might have unpredictable results in later versions.
Drag and drop is a useful mechanism to do stuff and can turn a complicated set of buttons or drop down lists into a simple elegant solution.
Wordpress has lots of scripts built in, which do a lot of useful functions. However, they are not enabled by default, which means that in order to get your script to work you need to enable them.