Both the Button and Site List widget serve as links to navigate through your app.
Buttons are bold, stand alone objects. You define the text on the button and set an internal or external link.
Site Lists, on the other hand, are a style of stacked buttons, meaning they fall one on top of the other when used in a series. Site Lists do the linking and titling for you, so all you have to do is select a page from a pull down list of all your pages and it will automatically link to that page. The text displayed on the button is automatically pulled from the page title of each individual page linked. Using the Site List widget, also gives you the option to display thumbnails for each linked page.
Using a Site List is an easy way to give your users easy access to all the points of interest in your app. It's also a nice visual alternative to a page - having many standard Buttons on a page can often look busy.
You can also choose to display icons in your site list.