XML node type issue

What is difference between these 4 types of XML nodes and when to use each of them?

• element
• text node
• comments
• instruction node

EDIT 1: whether my below understanding of text node is correct?

in the below sample, text node is abcd?

<foo>abcd</foo>

For a reference is really good this page W3Schools but if you are looking for some tutorials it probably won't be right one.

The best reference I've found for XSLT is 'XSLT 2.0 and XPath 2.0, 4th Edition' by Michael Kay. He's the spec lead for xslt 2.0 and xpath 2.0 and the implementor of the Saxon XSLT engine. It really is a great book - my copy has floated around the whole team and always has the answer if you look hard enough.

Another tip is to include the authors name in your xslt related search. He's quite active on the XSLT mailing list and including his name in the search term helps you jump straight to the answer you're looking for.