In the past, you could buy either a DVD or a flash drive with the OS on it. But this time, it's download only. As the above poster said, you could download it to an external drive and then install from there.
However, the OS is 3.4 (approx) gigs and hefty for storage. Once you download it from the App Store, it shows up in your purchased list as being available for download - even though you've already downloaded it once. You can put it on several (can't remember how many) computers without incurring additional charges - or re-download it at any time, should you need to do so.
So, essentially, Apple is storing it for you.