Signed

In programming languages a number is signed if it can hold both positive and negative values. Floating point numbers (floats and doubles) are usually always signed while integers can be signed or unsigned.

Signed integers are generally implemented using Two's Complement form which means that the highest order bit can be used to determine if the number is positive (0) or negative (1). Using unsigned numbers doubles the number of positive values that are available to integers while removing any negative values.

8 bit math example:
 * 00000111 = 0x07 => 7 signed and unsigned.
 * 111111001 = 0xF9 => -7 signed, 249 unsigned.