the biggest difference is an unsigned number cannot be negative (just as stated above)

but the benefit of usign an unsigned number is that you can have a greater range of positive numbers.

Tinyint: The signed range is -128 to 127. The unsigned range is 0 to 255.
Mediumint: The signed range is -8388608 to 8388607. The unsigned range is 0 to 16777215.
Bigint: The signed range is -9223372036854775808 to 9223372036854775807. The unsigned range is 0 to 18446744073709551615.

you get the point from there, a great use of unsigned numbers is for any column that has an auto_increment flag on it, since it will never be negative