first is the DocType seen in the first line of an HTML page. It looks like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
and tells the browser what standards to use to display the code.
Second is the Browser it's being displayed in, each browser has default styles for tags that can cause a lot of headaches. Making a page work across many browsers is one of the keys to good web design.
Third is CSS styles. CSS stands for Cascading Style Sheets and essentially a Stylesheet is a document made up of CSS code that tells the browser how to render the HTML code of the page. Think of it as an extension to the HTML language that gives you a whole bunch more flexibility.
I hope that helps.
