Short answer: The ISBN identifies a specific edition of a book. Match it to the one you need before buying.
What it is
An ISBN is a number that identifies a specific edition and format of a book.
Where to find it
Look near the barcode on the back cover or on the copyright page.
Use it
Compare the ISBN on the listing with the one your syllabus or teacher specifies.
Frequently asked questions
Is the ISBN always shown?
Not always. Ask the seller if it is missing.
Do reprints change ISBN?
Not necessarily. Check edition and year too.