HD Video: High definition video refers simply to any video with a higher level of image detail (resolution) than standard definition (SD). Where standard definition tellies display 480 vertical lines of picture, HD ones can display 720 (HD Ready), or even 1080 (Full HD). However, HD-capable displays will only display a higher definition picture when connected to an HD receiver.
Resolution: Describes the detail an image holds. Put very simply, the higher the numbers, the more detailed the image will be, regardless of screen size.
Refresh Rate: Measured in Hz, the speed at which the screen can refresh determines it's effectiveness at reproducing fast-paced action. Most basic sets refresh at 50 or 60Hz, generally fine, but some find it makes for eye strain when watching football, for example. As ever, this is more noticeable on larger screens. Sets with rates of 100Hz or more are now commonly available.
HD Ready: This refers to screens which are capable of displaying lower end high definition (HD) content, which is still far more detailed than regular, standard definition telly (720 lines as opposed to 480), but not as detailed as Full HD 1080p below.
Full HD 1080p: This refers to screens capable of displaying the highest quality of HD content (1080 lines). To do so, they need a resolution of at least 1920 x 1080.
HDMI (High Definition Multimedia Interface): The preferred interface for plugging HD devices into each other. It's digital, and carries both sound and picture information. As an aside, don't be hoodwinked into paying over the odds for 'gold-plated' cables; since it's a digital format, there's barely any difference between them and the cheapest HDMI cable.