OK, let me explain myself a bit.
Not all browsers display a page the same way.
This is because they interpret the various display blocks (eg div etc) in different ways
This is true even when the code is 100% standards compliant
This particularly applies to IE in all its versions
Furthermore, there are different standards to be compliant with.
You declare which standard your page will comply with by using a "Document Type Declaration" (DTD)
Of all the current browsers in common use today, Firefox is (IMO) the most standards compliant
Consequently, designers tend to review their designs as they evolve in Firefox
This will frequently leave glitches in other less compliant Browsers such as IE
Before signing of a page, it is therefore necessary to introduce "Fixes" to overcome the inconsistencies.
DTDs are a vital part of cross Browser performance and Standards in general.
If you don't include a DTD at all, most Browsers display in what is commonly called "Quirks Mode"
Quirks Mode is a way browsers display making allowances for Non Standards Compliance.
There is much information on Design and DTDs on the net just search in Google and you will find many good articles.
The W3C slant on DTDs can be found HERE
This is very comprehensive and will demonstrate the complexity of the issue.
I favour and recommend you to start using:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
This particular DTD says that the page is coded in HTML version 4.01 and references the W3C page that contains that coding standard definition.
All very complicated and a lot to absorb, but if you are to make your life a bit easier regarding cross browser campatibility, it has got to be understood to some degree.
I hope this cleared some of mist up for you and didn't just make things more complicated. The important thing to grasp, is that Browsers don't all display any given page the same. Further a particular Browser will display the same page differently if the DTD is changed. Hence it is VITAL, that the DTD matches the particular Standard that you used when making your pages. From experience, have found the DTD that I quotes above to be the best starting point because it relates to HTML v4.01 which is long established and widely understood.
One last point to further muddy the waters, a DTD that references a URL standard will have a different effect on a Browser to one that does not use a URL reference.