ISBN stands for "International Standard Book Number," and it is a ten-digit number identifying books being sold internationally.
Why have one? ISBNs distinguish one title (or one edition of a title) from one specific publisher. That particular ISBN remains with that particular title/edition, which helps in the book's marketing not only to booksellers, but to libraries, universities, wholesalers, and whatnot. Think of it as that edition's SSN.
What do its four divisions mean? (1) Location of the publisher (national or geographic group); (2) Publisher; (3) Title or edition of a title; and (4) Validation, which is a single digit shown at the end, providing the ISBN's accuracy. If you see an "X," think Roman numerals - here, it stands for 10.