ASCII stands for the American Standard Code for Information Interchange. The ASCII character set is simply the codes that represent each letter. So, when you type, you actually send a code to the computer, it's then interpreted by the computer and displayed as a letter. This kind of stuff is required for things to work otherwise, how would the keyboard manufacturers know what to get their keyboards to send to the computer? And the same goes when you print a document and all that. The ASCII character set is 128 characters / symbols represented by the numbers 0 to 127 (inclusive). After figuring out that this was a tad restrictive, the extended character set adds another 128 characters / symbols. (And nowadays, there's a whole heap more using Unicode, but that's another story).
ASCII Representation shows you the ASCII number of the keys you type and, as color can be displayed in red, green and blue increments of 0 to 256, a color representation of the key you are entering. Note that at the top of the page, two numbers are displayed - “key down” and “key submitted.” Key down changes with each key pressed, and represents the actual key pressed. Key submitted only captures the value submitted. So, pressing the “shift” key, for example, will register key down, but not key submitted. It will affect the value shown in key submitted. The text box only captures key submitted.
Key to ASCII doesn't really do a whole lot. It just converts an ASCII key to its corresponding character. There's also a full ASCII table displayed and you can copy your selected symbol to the clipboard if you want to.
Given that everything is now in UTF-8, we've added unicode by block to allow for the ability to select a unicode block and display the characters contained within.