Decimal (base 10)
This is the commonsense math with which you are acquainted. Constants without other prefixes are assumed to be in decimal format.
Example Code:
n = 101; // same as 101 decimal ((1 * 10^2) + (0 * 10^1) + 1)
Binary (base 2)
Only the characters 0 and 1 are valid.
Example Code:
n = 0b101; // same as 5 decimal ((1 * 2^2) + (0 * 2^1) + 1)
Octal (base 8)
Only the characters 0 through 7 are valid. Octal values are indicated by the prefix "0" (zero).
Example Code:
n = 0101; // same as 65 decimal ((1 * 8^2) + (0 * 8^1) + 1)
It is possible to generate a hardtofind bug by (unintentionally) including a leading zero before a constant and having the compiler unintentionally interpret your constant as octal.
Hexadecimal (base 16)
Valid characters are 0 through 9 and letters A through F; A has the value 10, B is 11, up to F, which is 15. Hex values are indicated by the prefix "0x". Note that AF may be upper (AF) or lower case (af).
Example Code:
n = 0x101; // same as 257 decimal ((1 * 16^2) + (0 * 16^1) + 1)
Notes and Warnings
By default, an integer constant is treated as an int with the attendant limitations in values. To specify an integer constant with another data type, follow it with:

a 'u' or 'U' to force the constant into an unsigned data format. Example: 33u

a 'l' or 'L' to force the constant into a long data format. Example: 100000L

a 'ul' or 'UL' to force the constant into an unsigned long constant. Example: 32767ul