Please dirrect me -- headers ????

This isn't necessarily PHP specific, but I'm looking for more information on HTTP headers. I'd like to know what headers I should send and why. I want my code to send out w3c compliant HTML. I'm confused on exactly what headers do, and the difference between HTTP headers and information contained within the <HEAD> tag. Multiple google searches have gotten me only more confused. Someone please send me in the right direction :) on an a possible similar note, I have a domain name www.mydom1.com and when the user goes to it, I want it so say www.mydom.com in the browser address bar. Can I do this by sending certain headers? (I have both domains pointing to the same server) ... should/could I do something like this through PHP or is there something I can set in Apache to do the same thing? Simply confused, - Anthony