Every major cover change denotes a new edition (1st, 2nd, etc..). The content of a book may or may not have changed much due to editing, revisions, or updates, and sometimes it is just the cover that has changed. You can read each book's description and the changes will be noted. The edition enumerating is for cataloging purposes, especially on sites such as Goodreads.
This guide is to help you, Dear Reader, from accidentally purchasing the same book twice.