I'm trying to learn about serving a page as text/html or application/xhtml+xml.... Where is this distinction made? On a server setting? In the meta element's content attribute?

I'm not even sure I'm using the right terminology here..

I really don't understand this aspect of mark up at all. I found articles online that say serving xhtml as text/html is bad and it lists reasons, but I'm pretty sure I do that and my pages look the way I want, tested in a number of browsers (with some css tweaking of course).