Screen resolutions
Most people who have had any invlovement with website production will have heard of them, but not everyone understands what screen resolutions are or how the effect web design visually and technically.
Size isn't everything
These days monitors can often be set to screen resolutions of 1440x900 pixels as easily as 640x480 with various other settings in between, irrespective of the physical size of the screen. This has long caused problems for web designers as, unlike printed media where the size can be precisely dictated, the viewable area available can and often does vary from one website visitor to another.
Only this very day one of my clients asked about putting some graphics on "all that space on the right" and I had to remind him that for a significant propertion of his website's audience there is no space on the right. It's true that the 800x600 setting is becoming less common than a couple of years ago when over 50% of all users globally used it, and now represents probably somewhere in the region of 30% but 30% is still a significant figure.
I'm not sure where they get their figures from but these look feasible to me: http://www.w3schools.com/browsers/browsers_stats.asp
Still confused?
So far I've prattled on as though everyone reading this knows what screen resolutions are, but not everyone does. Over the years I've had many conversations with people who just couldn't understand that increasing a screen resolution makes everything on a screen smaller and decreasing it means everything looks bigger but you get less of it.
It's not the easiest thing to explain and they best way to get to grips with this concept is to play with it. To that end, if you're a Windows/PC user right-click on your desktop and select 'Properties' then select the 'Settings' tab. You should then be able to twiddle your screen resolution settings to your heart's content. Providing you don't inadvertently set your screen so that your dialogue boxes can't be seen, leaving you in a world of perpetual PC-blindness you should be able to put things back the way you want. You might discover that you've been missing a high-res world all this time.
If it's good enough for the BBC...
The BBC run Europe's biggest and busiest website and will have had countless committees pawing over every aspect of usability, and the fact that all of their web-pages are no more than 770 pixels wide says a lot.