MySQL: bigint Vs int
The difference is purely in the maximum value which can be stored (18,446,744,073,709,551,615 for the bigint(20) and 4,294,967,295 for the int(10), I believe), as per the details on the MySQL Numeric Types manual page.
Incidentally, the use of (20) and (10) is largely irrelevant unless you're using ZEROFILL. (i.e.: It doesn't actually change the size of the number stored - that's all down to the type.)
However, in practical terms it should be noted that you're not likely to hit either of these limits any time soon, unless you're a really active blogger.
The only difference is the range of the type. INT is a 32-bit long while BIGINT is 64-bit long, therefore it can store much larger numbers like 123456789123456789 (which cannot be stored as INT).
Here's a full list of MySQL integer types: http://dev.mysql.com/doc/refman/5.0/en/numeric-types.html