The first thing that you'll notice about widgets is that the are somewhat similar to a macro expansion, such as C's #define. Your source HTML is a simple

<button dojoType="Button" id="foo"> Click me </button>

and yet a pretty blue button shows up, and when you look at the generated DOM, it's a complicated tree of DOM nodes with a lot of absolute positioning and background images.

See? Click me

But, that's not all there is. For each widget, besides the visible manifestation, there's also a pure javascript object that manages that generated DOM tree.

In the above case, the generated javascript object is called, unsuprisingly, "foo". You can get it by doing:

var myButton = dojo.widget.byId("foo");