HTML = Hyper text markup language.
XHTML = Extensible hyper text markup language.
The most important differences between HTML and XHTML are:
- XHTML elements must be properly nested
- XHTML elements must always be closed
- XHTML elements must be in lowercase
- XHTML documents must have one root element.
I found a website that was really helpful with this research, http://www.w3schools.com/