For INT and other numeric types that attribute only specifies the display width.
See Numeric Type Attributes in the MySQL documentation:
MySQL supports an extension for
optionally specifying the display
width of integer data types in
parentheses following the base keyword
for the type. For example, INT(4)
specifies an INT with a display width
of four digits. This optional display
width may be used by applications to
display integer values having a width
less than the width specified for the
column by left-padding them with
spaces. (That is, this width is
present in the metadata returned with
result sets. Whether it is used or not
is up to the application.)The display width does not constrain
the range of values that can be stored
in the column. Nor does it prevent
values wider than the column display
width from being displayed correctly.
For example, a column specified as
SMALLINT(3) has the usual SMALLINT
range of -32768 to 32767, and values
outside the range permitted by three
digits are displayed in full using
more than three digits.