"Measured diagonally" means that the screen is measured from corner to corner across the face (crossing both width and height). This makes a lot of sense with squarish screens, where doing so means that comparing measures gives a good idea of difference between screens. But consider the impression '19" screen' gives between a squar[ish] and a wide screen when you look at them side-by-side:
--Apologies for the dirty screens (these are newly acquired, and haven't been cleaned yet), and none for the flash-I like it.--
The number is an accurate measure in either case, it's still a measure across face (both width and height), but the impression of the different forms of these screens is, well, different. The squar[ish] screen (Dell) really isn't too much higher (and therefore, narrower) than the "wide" screen (Compaq); nevertheless it is not very useful for things like putting two documents side-by-side on the same screen; the wide screen contrariwise fits two documents besides one another comfortably. Thus the wide screen is perhaps the more useful for many practical purposes.
Now as far as Windows users go about doing that manually every time they have to adjust a Window, and put up with the time that must take, is beyond me: I have used Linux for years, only recently getting a Lenovo with Windows (and note removing it), which is nice for Excel and OneNote, but in almost every other way inferior (in my view); with Linux one can set screens to snap to attention, to fit side-by-side together on a screen just by default. It's beautiful.
Well a little comparing of screentypes to get the nontechie's juices flowing, and a bit of plug for Linux systems, and I'm about off. OH! And warning: Ubuntu with desktop "Unity" won't help you in the Windowing game, being built to prefer only a single Window displayed at a time, apparently targetting tablets. Who knows if they will repent in time.