An International Standard Book Number is a unique 13 digit number used to identify publications. A different ISBN is required for each edition of a publication, including ebooks. While there is no legal requirement (in the UK) for a publication to have an ISBN, most booksellers require one to integrate with their processing and ordering systems.
Getting An ISBN
Nielsen are the UK ISBN Agency. You can order ISBNs in blocks of 10, 100 or 1000. There is currently no option to purchase a single ISBN. The standard processing time is 10 working days, but there is fast-track option which reduces this to 3 days. The Services for New Publishers page on the Nielsen website has more information on ordering ISBNs.
US publishers can order ISBNs from Bowker. ISBN agencies in other countries can be found via the International ISBN Agency.