If a browser sees a full DTD as the FIRST element of a document, including the W3C URL for the details, then it renders the page in "standards" mode. Because standards are still relatively young, there is some variation from one browser to another, but it's usually minor.
But if a browser sees no DTD, or a partial DTD, then it goes into "quirks mode", which essentially means rendering the page the wrong way, but the way we were used to up until version 6.
What this all means in specific detail is way beyond the scope of one thread. But I thought it would be very useful to have this explanation and a set of references in one spot, to get people started out on the right foot if this is all new to them. More and more, you will see these two modes mentioned in threads.