dojo.byId(String id)

This function is a synonym for document.all.id in IE or document.getElementById(id) in standard DOM. So you can say:

If document.getElementById() works in all modern browsers,why use this? Sadly,bugs persist in Internet Explorer which make getElementById() unstable in some common scenarios. Also,dojo.byId() is easier to type.

dijit.byId(String id)

Because of the almost-identical spelling,this method is commonly mixed up with dojo.byId(String id). Here's the difference: dijit.byId() returns a Dijit widget instance - technically,an instance of dijit._Widget or one of its subclasses like dijit.Toolbar or dijit.TreeNode.

So here's the way to remember it:

• If you need a DOM Node,use dojo.byId. You need a DOM node anytime you call a standard browser function,change a standard property,etc.
• If you need a widget,use dijit.byId. From it you can access all of the attributes,methods and extension points listed in Part 2.

Or for you who think in terms of functions (e.g. LISP programmers?):

var myDomNode = dojo.byId("foo");
var myWidget = dijit.byId("foo");
console.debug(myDomNode == myWidget.domNode);  // true
console.debug(dijit.byNode(myDomNode) == myWidget); // true