An ISBN is a unique Identifier for a Book. It stands for International Standard Book Number. A variation is EISBN for eBooks but this is not always followed.
All commercial books require an ISBN number. Others may have one. Generally hardcover and softcover Books have a different number and they are usually a different edition. Some recommend that the eBook version have its own number. Some even suggest that each eBook format have its own number. This can get expensive since the numbers are not free. Self published books may be able to get by with just a unique identifier as required by ePub. Audio books are also candidates for an ISBN number.
Books can be searched by using their ISBN number directly from this wiki using this page