Apple historically has sold OS X through its Apple Store and third-party vendors on physical media and included a classical End User License Agreement that limited users to one installation of the software on one Apple-branded machine. Given that people own multiple Macs, Apple introduced family pack licenses with installations for up to five systems. This offered individuals with multiple Macs a discount for upgrading.
The availability of these past options have confused people about the licensing for Apple's latest OS X Lion, since it's available as an download from the Mac App Store instead of being distributed on DVD. People are also confused because they wish to redownload OS X to create an installer DVD so they can quickly install on multiple machines, but see it listed in the store as already being installed on their systems.
The simple rule to keep in mind is, as Apple mentions in the first statement in its App Store FAQ, that "Apps from the Mac App Store may be used on any Macs that you own or control for your personal use."
This means the license for use revolves around the purchase connection with your Apple ID, and not around a combination of a license key and machine.
As with other software distributed through the App Store, Apple has also adopted this usage license provision for its operating system software. So whether you own one Mac or 20, you can use your Apple ID to make one purchase of OS X Lion and then install it on any Apple-branded Mac currently running OS X Snow Leopard or Snow Leopard server.
This means you only have to pay once for this purchase, instead of paying for each system you own as was the case with prior versions of OS X.
The same licensing also applies to OS X Lion distributions on physical USB media. The only situation when the licensing differs is for installations that shipped with a new Mac. You are only allowed to use it on the system on which it was shipped, and not clone or copy it to other systems.
In addition to allowing multiple Macs to run the software, the license offers a single Mac to run up to two additional copies of OS X in virtual machines using programs like Parallels Desktop and VMware Fusion. This means you can create a sequestered and separate Mac OS X installation within your current Mac to test software or demonstrate a point about the operating system without using your main OS installation.
These license allowance changes by Apple are for the use of its software for personal use only. There are differences for those who wish to use the software in volume license. If you think you have a special situation and are curious about the details of the license, you can check out Apple's product agreements Web site.
So far there are no details about whether this agreement will change for Apple's upcoming OS X 10.8 Mountain Lion, but since it will also be distributed through the Mac App Store, it's likely the agreement will be similar.