We are all familiar with numbers in the decimal system, where each place has a value 10 times the place immediately to the right. Thus the number 12,345 in decimal is 5 units plus 4 tens plus 3 hundreds plus 2 thousands plus 1 ten-thousands, or twelve thousand three hundred and forty-five.
Any number is made up of combinations of the ten digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.
Now it doesn't have to be this way - we don't have to count in tens. In the hexadecimal (hex = 6, deci = 10) system we count in sixteens rather than in tens. Instead of place values being 1, 10, 100, 1,000, 10,000 etc (reading from the right, equivalent to 1, 10, 10 x 10, 10 x 10 x 10, 10 x 10 x 10 x 10 etc) we can have place values of 1, 16, 256, 4096, 65,536 etc (equivalent to 1, 16, 16 x 16, 16 x 16 x 16, 16 x 16 x 16 x 16 etc).
Thus, on the hexadecimal system, the number 12,345 would have the value 5 units plus 4 sixteens plus 3 two-hundred-and-fifty-sixes plus 2 four-thousand-and-ninety-sixes plus 1 sixty-five-thousands-five-hundred-and-thirty-sixes, or 74,565 in decimal numbers.
There remains one problem. We've seen that the decimal system requires the ten digits 0 to 9. In the hexadecimal system we need 16 digits, so we add on the first six letters of the alphabet i.e. 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F. A represents 10, B represents 11 down to F representing 15.
For instance, the hexadecimal number CAD has the value 13 units (D = 13) plus 10 sixteens (A = 10 in the 16's column) plus 12 two-hundred-and-fifty-sixes (C = 12 in the 256's column) which is 13 + 160 + 3072 or 3245.
Hexadecimal is used because it makes binary numbers (the basis of computing, where you go up in 2's rather than 10's or 16's) more intelligible. For example, 10101011 in binary is AB in hexadecimal and 171 in decimal.
https://simple.wikipedia.org/wiki/Hexadecimal_numeral_system
