When books and journals are published, they're given a unique number by the publisher to identify them.

Books are given an ISBN - International Standard Book Number. It is always 10 digits long and it can have a combination of numbers and letters or just numbers. Example: World anthropologies : disciplinary transformations within systems of power is 1845201914 for the paperback edition. Different editions have a different ISBN.

Journals are given an ISSN - International Standard Serial Number. These are 8 digits, with a dash after the first 4 digits. It can be a combination of numbers and letters, but it's usually just numbers. Example: African Journal of Ecology is 0141-6707.

These numbers are important for ordering books/journals and for ensuring that you have the right one when you're trying to locate something.