What’s the difference in int(11) and int(11) UNSIGNED?

An UNSIGNED type cannot be negative, but on the other hand it has twice as large a range for the positive integers. The types TINYINT, SMALLINT, MEDIUMINT,
INT and BIGINT all have signed and unsigned versions.

For INT the ranges are defined as follows:

Type          Storage         Min           Max
INT                 4 -2147483648    2147483647
INT UNSIGNED        4           0    4294967295

The signed and unsigned types take the same storage space (4 bytes for INT).

See the documentation for more details.