I was writing a code for my friend who wants it to be sent to people through html.
I am new to this.
So i wrote a code and gave him after seeing which he told me i had to write it either in HTML or in XHTML.
Mixing both would trigger spam filters.
What is the difference between pure html and pure xhtml?

XHTML is a modern version of HTML - it is more regulated and therefore; cleaner.

Just stick to XHTML.

http://validator.w3.org/ can assist you in validating your code depending on the doc type you have (HTML/XHTML-Transitional/XHTML-Strict) etc.

edit: oh and the stuff about spam-filters.........my answer is !?!!?!??!?!?!?!?!??!?!

Exactly. It doesn't matter what 'version' of HTML you use, it shouldn't trigger spam filters at all.