In the olden days, every browser rendered things differently. Complicating matters, most of these browsers paid only the slightest attention to the standards set forth by the W3C, so web designers either spent a whole lot of time and money figuring out how to render their pages correctly in all browsers, or just wrote their pages for one browser only. Now that the most popular browsers have come around to following the standards, a whole lot of established web sites are having to make some changes to keep up.
Eric Meyer has a good write-up on this topic, wherein he covers the differences in the older browsers, how to keep some modern browsers in "quirks" mode (which causes the browser to render pages just as their non-standard compliant predecessors did), and lists some additional resources for the tragically interested.
For anyone updating old web code, this is essential reading.