Hexadecimal is a base 16 numbering system, in which each digit of a number are represented by 16 possible characters. These chracters are 0-9 and A-F. Hexadecimal is widely used when representing binary numbers, as they can perfectly represent a byte using only two digits.
Hexadecimal is an excellent way to represent large decimal numbers as you need less digits to represent the same number. For a classic 32 bit integer, you only need 8 hex digits to represent the maximum value, as opposed to the 11 required when using classical base 10 representations. Even numbers are nice!
A traditional decimal number is the sum of digits multiplied by powers of ten. Hexadecimal is the same, but with powers of sixteen.
f0a183b16 to decimal:
f0a183b16 = 15×166+0×165+10×164+1×163+8×162+3×161+11×160
f0a183b16 = 25231997910