An ISBN is the International Standard Book Number: a unique identifier for one specific edition, format and printing of a book. Two copies with the same ISBN are the same book. Two copies with the same title might not be.
Where to find it
Look on the back cover directly above or below the barcode, on the copyright page inside the front matter, or on your course syllabus — most instructors list it. Bookstore course-material pages almost always show it alongside the title.
ISBN-10 versus ISBN-13
Books published before 2007 typically carry a 10-digit ISBN; newer ones carry 13 digits, usually beginning 978 or 979. They refer to the same book, and most retailers accept either. Hyphens are cosmetic — you can type them or leave them out.
Why title searches mislead you
Publishers release new editions every two to three years, and each one gets its own ISBN. A title search returns all of them mixed together, sorted by whatever the retailer wants to sell, and the cheap listing you found may be an edition your course does not use. Searching by ISBN removes that ambiguity entirely.
Formats have separate ISBNs too
Hardcover, paperback, loose-leaf and ebook versions of the same edition each carry a different ISBN. This matters when a course requires a specific format, and it matters for resale — loose-leaf copies are worth substantially less on buyback because they cannot be resold as easily.
A caution about bundles
Many introductory courses assign a bundle: the textbook plus a single-use online access code. The bundle has its own ISBN. Buying the standalone book by its own ISBN is cheaper, but if your homework lives behind the access code, you will still need to buy the code separately — and codes are rarely cheaper on their own. Check what your course actually requires before choosing.