Microsoft's Windows 10 operating system will be available in retail stores in the form of USB drive versions, you can already pre-order these on Amazon. The Windows 10 Home and Windows 10 Pro USB drive sticks go for $119.99 and $199.99, respectively.
Ditching DVD drives makes sense as optical discs are no longer widely used and a lot of systems, especially thin laptops, no longer have disc drives.
The preferred method remains digital distribution of course, if you have Windows 7 or Windows 8.1 you'll be able to upgrade to Windows 10 for free. The launch date is July 29th but it's going to take a bit longer until everyone gets it, if you accept Microsoft's free upgrade you're placed in a queue and Microsoft will be doing daily rollout cycles so they have time to adjust things in case problems arise.