On January 1st, 2007, something momentous will occur within the book publishing world: the standard for ISBN numbers will change from ten digits to thirteen digits.
Why will this happen? The short and simple answer is that the current system, based on ten digits, is a finite one and will run out of possible number sequences fairly soon. According to the web site of Random House, one of the largest (if not the largest) publishers in the world, "The international ISBN agency expects to run out of some available 10-digit ranges as early as mid-2007."
As harrowing as all of this may sound, the simple fact of the matter is that many computer systems in the book world- especially the POS ("point-of-sale" or "cash register") computers used to ring through your book purchase at your local bookstore- already use a system which "reads" the ten digit ISBN as a thirteen digit one: the EAN Bookland barcode. The Bookland write-up is an excellent and informative source explaining this process in detail; to put it in a nutshell, EAN barcodes look almost the same as the ten digit ISBN, with the three additional digits "978" added at the front.
The astute reader may ask, what's the use? If all the ISBN people are doing, effectively, is adding "978"- to existing ten-digit ISBNs, why bother? The finite number of sequences will vanish in the same amount of time, with or without the "978"- addition. In fact, the "978" prefix is currently how barcode scanners, which are normally accustomed to 12 or 13 digit Universal Product Code barcodes, are programmed to recognize the information in a ten digit ISBN number. After the ISBN-13 transition in 2007, 978- is only the first initial sequence. When all available number blocks are taken up, 979- sequences will be issued. (Presumably, with 980- and 981-, etc. to follow, although such occurences are likely to be decades away as the 10-digit ISBN system has been in use for the last several decades.) This is akin to the telephone industry's issuing of 888 toll free numbers when all possible combinations in the 800 range were taken, followed by 877, then presumable 866, 855, etc.
One final interesting fact to note is that, when the 979- prefix switch is made, publishers will not necessarily get the same identifying blocks issued to them. (For example, in the current ten digit ISBN system, 0-385 is one of the identifying blocks owned by Random House. So, 0-385-XXXXX-X (where "X" represents any digit) identifies a book published by Random House. In the ISBN-13 transition, this will become 978-0-385-XXXXX-X. Once 979- blocks are issued, the sequence 979-0-385-XXXXX-X may not necessarily be given to Random House, but to another publisher entirely.
Information taken from the websites for Random House and Random House of Canada, http://www.randomhouse.com and http://www.randomhouse.ca, and for the Book Industry Study Group at http://www.bisg.org/isbn-13/.