Chiming in with another shameless plug; I recently wrote an article where I somewhat desperately try to explain some of the concepts of WiX in a more easily-understood way than the others. No offence.
http://www.optimalbpm.se/wiki/index.php/WiX
It is meant to be read before reading any tutorials, because it specifically tries to address some of the things people always seem to misunderstand as a result of well, reading those tutorials. And especially the documentation. At least I did.
Also, there is another article, a little bit more advanced, that has an angle to those distributing script(like Python etc)-based systems, and why one in not only those, but all cases should use Paraffin rather than Heat.
http://www.optimalbpm.se/wiki/index.php/Wix_and_scripting_languages