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All the SQL dialects I’ve seen use to either allow or to require to specify an integer argument for some of the data types they support when defining a table. But I haven’t managed to find any comprehensive information (at least for MySQL and SQLite) about what exactly do these numbers mean and how to chose them adequately…
If you mean the notation like INT(11), VARCHAR(255), then normally it’s the length of the values stored (or retrieved) from the table.
http://dev.mysql.com/doc/refman/5.5/en/numeric-type-overview.html
http://dev.mysql.com/doc/refman/5.5/en/string-type-overview.html
There’s an “M” in parentheses after name of every type which supports it.
Also, keep in mind that these numbers may affect both storage and selection length. For example, if you define a column as VARCHAR(100), the actual space reserved for each value will still be 255. But the values you retrieve in SELECT will be trimmed down to 100 characters. And if you define a VARCHAR(256), then it reserves up to 65535 characters, if I remember correctly.
The same with INT(5). It still reserves space for storing values up to 2147483647 (max value for signed integers), but trims the input/output to 5 digits.