HTML and XHTML

After going through all of the tutorials, I learned some stuff that we didn't go over in class. Pretty much, XHTML and HTML are very similar. XHTML is a little more strict with its coding. All tags need to be lowercase and closed. The only thing that confused me on the tutorial was how they closed the tags with " /". We didn't use the extra space in class, but they recommended you do that. When I ran the XHTML validator, it found that I didn't have a Doc Type and that I didn't put spaces before my closing '/' in br/. Should ask whether to put it or not?