Straight from http://www.mysql.com/doc/en/Column_types.html :
INT[(M)] [UNSIGNED] [ZEROFILL]
A normal-size integer. The signed range is -2147483648 to 2147483647. The unsigned range is 0 to 4294967295.
BIGINT[(M)] [UNSIGNED] [ZEROFILL]
A large integer. The signed range is -9223372036854775808 to 9223372036854775807. The unsigned range is 0 to 18446744073709551615. Some things you should be aware of with respect to BIGINT columns:
All arithmetic is done using signed BIGINT or DOUBLE values, so you shouldn't use unsigned big integers larger than 9223372036854775807 (63 bits) except with bit functions! If you do that, some of the last digits in the result may be wrong because of rounding errors when converting the BIGINT to a DOUBLE. MySQL 4.0 can handle BIGINT in the following cases:
Use integers to store big unsigned values in a BIGINT column.
In MIN(big_int_column) and MAX(big_int_column).
When using operators (+, -, , etc.) where both operands are integers.
You can always store an exact integer value in a BIGINT column by storing it as a string. In this case, MySQL will perform a string-to-number conversion that involves no intermediate double representation.
-',+', and `' will use BIGINT arithmetic when both arguments are integer values! This means that if you multiply two big integers (or results from functions that return integers) you may get unexpected results when the result is larger than 9223372036854775807.
Looks to me like you would want to use BIGINT when you knew you were going to have a ton of entries in the future. Otherwise, it probably doesn't matter ... I use INT usually for auto_increment fields.
