From David Bolton,
Your Guide to C / C++ / C#.
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Definition: Hexadecimal is base 16 arithmetic where each digit is a value from 0 to 15, rather than the 0-9 of base 10. The decimal numbers after 9 are represented by the letters A to F where

A₁₆= 10₁₀
B₁₆= 11₁₀
C₁₆= 12₁₀
D₁₆= 13₁₀
E₁₆= 14₁₀
F₁₆= 15₁₀

Its a concise way of expressing numbers. 255₁₀ = FF₁₆. FF = 15*16 + 15. Each digit to the left is 16 times the previous one 400₁₆ = 4*256 = 1024.

In C, C++ and C# numbers are represented as hexadecimal by putting 0x in front. E.g. 0x300 is the same as 768.

Hexadecimal is a very convenient way of showing binary numbers as every 4 binary digits can be shown as one hexadecimal digit.

Glossary:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Also Known As: hex,base 16

Examples: I converted 247₁₀ into hexadecimal. The answer was F7₁₆.
1011₂ = B₁₆