HD? Full HD?? XHD???All described here!

You must be going nuts, hearing all these terms everywhere-TVs, GPUs, Disk players and so on.
What does all this mean?Read on.
When you watch your regular cable TV or VCD, the quality of picture that you see is in SD or Standard Defenition.This has 640 vertical 'lines' of small dots called 'pixels' and 480 horizontal ones[The number of such lines is the called the resolution of the TV,in this case-640x480].This makes a good quality picture but not as well defined as what we see around us.Let us keep this as the basis for comparing the others listed below-
HDTV [also-HD ready]
This has more lines- 1080 vertical lines and 768 horizontal ones[1080x768].This provides more detail in what you watch, about 3.5 times more detailed than what SDTV offers.This is still not as good as what our eyes can see.
Full HD
This comes with 1920 vertical lines and 1080 horizontal ones[1920x1080].This can show images that are 5 times more detailed than SDTV.You can see snowflakes, grains of sand and other fine things much more clearly.This is very close to all the eye can see, but still is not all.
Extreme High Defenition[XHD][2560x1600]
This is THE best available right now.This has 2560 vertical lines and 1600 horizontal lines for, true to its name, extreme detail!This offers about 7.5 times the detail of SDTV!!!As of now, this is mainly for over-the-top computers with latest graphics cards, software etc.

Its just like using a thick tipped felt pen and a fine tipped pen or pencil for a drawing, the pencil's demarcations will be a lot more clearer than that made by a felt pen.The pencil image will be sharper and clearer.
Basically what all this means is your pixel size will be different.the smaller the pixels, the finer the image you see.
Higher resolutions enable better contrast between colours, sharper, more precise images and better detail.Movies and games appear more realistic, engrossing etc.
I hope you got an idea of what all this means now!