Fundamentals

Simply said, your site must look good, load quickly, function properly, fulfills its' purpose, be up to date, and stay sticky. It must also provide you with feedback and visitor analysis.
To accomplish these things you must first define your audience and your site type. During this process you can use these sections as a reference source to help you understand the developers goals, and your own.

A good place to start is an understanding of bandwidth and browsers.

Bandwith

How fast is the connection? 14.4, 28.8, 33.6, 56K modems , ISDN, ADSL, cable modems.  

Browsers

Netscape, Internet Explorer, AOL? And which version, 2, 3, 4, 5 ?  

Screen size and colors

640x480 at 256 colors OR 800x600 at 16,000,000 colors.

14", 15", 17", 19" monitors, or is it a laptop?
This will affect how your graphics and fonts is displayed. 

Other fundamentals to consider are