Many authors confuse an ISBN number with a barcode. An ISBN number is required (although you may not need to purchase one as discussed below). However, a barcode is only required if you are distributing a physical or printed copy of your title.
An ISBN is a unique identifier assigned to your book that ensures that people ordering your book are receiving the intended publication. Part of the number signifies the publisher, while the rest is associated with that particular title, author and version.
A barcode does essentially the same thing (uniquely identifies your book) but in such a way that retailers can enter the code into their proprietary databases. This allows checkout scanners to properly associate a price with the book and gives merchants an efficient method to track their inventory.
So the simple answer to whether you need an ISBN – yes. The question should be where and in what formats do you want to make your title available? You will need a barcode (excluding ebooks) and an ISBN number for each distinct publication.