What is anchorNode , baseNode , extentNode and focusNode in the object returned by document.getSelection?

If I make a selection in a html page and I do :

var a = document.getSelection()

I get an object with four properties :

1. anchorNode
2. baseNode
3. extentNode
4. focusNode

the values of first three is the same i.e. the text that I have selected but how are they different and which one to use?