What does High Definition ready mean?

It's an industry initiative to improve consumer understanding and help prospective buyers know what they are getting for their money.
Put simply, if the TV has an HD Ready label it SHOULD support:

480i/p
576i/p
720p
1080i

What this usually means is that the screen will 'accept' and display images of the above format. Usually it means that the screen will upscale and downscale images where necessary. e.g. 1080i will be downsized to fit the native resolution of the screen in question.

The unit SHOULD also support HDCP over HDMI - which will be crucial once High Definition broadcasting and copy protected material becomes available over the air and via cable / satellite providers.

Screens

 CRT - Cathode Ray Tube: "A CRT works by moving an electron beam back and forth across the back of the screen. Each time the beam makes a pass across the screen, it lights up phosphor dots on the inside of the glass tube, thereby illuminating the active portions of the screen. By drawing many such lines from the top to the bottom of the screen, it creates an entire screenful of images."
-Standard Televisions: Everyone's probably got one somewhere. Disadvantages include low definition (image blurring), crosstalk and line twitter / unstable images. CRT is considered 'older' technology these days and are generally undesirable due to their bulky nature. However, there are many excellent CRT screens being produced, and in fact High Definition CRT's are still very popular in the US.
-Front Projectors: Uses a series of Red, Green and Blue guns to fire a convergent image onto a projection screen. Generally regarded as one of the best home projection methods. However they are very expensive, massively bulky and require professional levels of maintenance and calibration.

 Plasma - Gas Plasma: "Plasma technology is different from that used in other display systems in that red, green and blue lights are created in every pixel, reducing the need for space. Charged electrodes between glass panels cause tiny pockets of inert gas to change a state of plasma. This process causes UV light to be produced, which in turn reacts with the red, green, and blue phosphors in each pixel to produce visible light.

Unlike traditional displays, where the image is scanned across the screen, in plasma displays all pixels are "lit" at once. Having no electron beam, back lighting or light polarisation, the image is inherently sharper and brighter. Perfect from edge to edge. "
- Plasma televisions: Offer sharp and vivid images with good colour registration. Disadvantages include poor screen life (screen burn susceptible with images static for long periods of time), images can lack contrast and many early screens had problems registering blacks well causing a lack of depth in most images.

 LCD - Liquid Crystal Display:
- standard Televisions: The current darling of the high-street, boasting excellent colour fidelity, but poor contrast.
- Front Projectors: Use LCD panels mounted in series, light is projected through panels onto a projection screen. Advantages include, great image size, colour fidelity and clarity. Disadvantages include: low brightness in ambient lighting conditions, poor contrast on lower end models.

 DLP - Digital Light Processor: Technology uses an array of tiny spinning mirrors to reflect light at specific times in order to make an image. Far better explanation of the technology can be found here.
- Rear Projection TV: DLP source image is bounced off a mirror onto the reverse of a forward facing screen. Advantages, outstanding brightness, excellent viewing angles. Disadvantages, colour wheel used in DLP tech can cause headaches.
- Front Projectors: As LCD but using DLP tech. Advantages, outstanding brightness and contrast - even in ambient lighting conditions. Disadvantages - see colour wheel headaches above.

Newer Screens

SED (Surface-Conduction Electron-emitter Display) from Toshiba. Benefits include sharper, brighter, faster displays with the low profile sizes of LCD and Plasma panels.

SXRD (Silicon X-tal Reflective Display) - Projection specific technology used in Sony's high-end 'Qualia' projection systems. More info here.

3-Chip DLP - Actually present / past technology this but prices are dropping, currently used mainly for HighEnd cinema projection systems. This system uses 3 DLP chips for Red, Green and Blue (RGB) elements of the image. Standard DLP projectors only use one chip which essentially produces a monochrome image, colour is added by use of a colour wheel in front of the assembled image. This can cause headaches as the colour 'spin' speed can be detected by the human eye and brain. This is not a problem with a 3 Chip solution, as no colour wheel is involved. More info at the bottom of this page.

 

What about the games?

The new generation of consoles are set to push HD gaming into the public's perception at last. Whilst both the PS2 and the X-Box supported both 720p and 1080i, they lacked the muscle to make use of them across the board. With Xbox 360 and PS3 boasting heaving masses of powerful graphics power, most games are expected to be presented in 720p.