First, you have to think back to simple computers. A person hit a key, a signal went to the computer saying what key was hit. All keyboards had to send the same signal - otherwise if you switched your keyboard, it wouldn't work any more. So ASCII was created in the 1960s. ASCII stands for the American Standard Code for Information Interchange.
So there were of course codes for every letter of the alphabet - both uppercase and lowercase - as well as all of the numbers and symbols on a keyboard. There were also codes for the various arrow keys, and the space bar, the return/enter key, and so on.
In addition, special key combinations (usually using the ALT key) were created for making other characters like the degree symbol, the various European letters with tildes and accents and so on. Those combinations also got assigned ASCII codes.
In ASP, you would want to check ASCII codes if for example you were looking for a carriage return in a comment field someone filled out, or something similar. You simply use the ASC function to determine the ascii value of the character and see if it's equal to 13. The ASCII character for a space is 32. The other characters before 32 are all the special characters like escapes and arrow keys that you wouldn't really use on the web or in ASP programming.
For the standard set of keyboard characters that someone is likely to type in to your ASP forms, use the
ASP and HTML ASCII Chart
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