The short story is that it is a 12 digit number, usually embossed or engraved at the very stern. It is often on the transom, and under the lip of there is one. It is broken up into three secionts; the first three are the Manufacturers Identificatio Code. The next five are the unique hull number. The last four are the manufacture date and model year.
If you are interested in determining who built a particular boat, enter the first three digits of the hull number here. They should be all alphabetic characters.
Ther are actually multiple formats for the last four digits, you can read more about that at the above link. Supposedly since 1984, the format is AXYZ, as follows:
The reason for the redundancy in build vs. model year is to distinguish model year boats that were built at the end of the previous year, which is common.