Rights and sites
When you buy an app on the Mac App Store, you’re getting the rights to run that program on any Macs you own and operate, for your personal use. Basically, if your household has a half-dozen different Macs, including desktops and laptops, you can buy a copy of Gratuitous Space Battles and play it on every single one of them. Consider a purchase of consumer software via the Mac App Store to be a bit like buying a household site license for the app.
The situation is slightly different for apps that are considered commercial or professional in nature. For apps that fall into this category—Aperture’s a good example—the Mac App Store license says that you essentially can install that item on computers you use or on a single computer shared by multiple people. Basically think of it as a one-seat license for a pro app.
In other words, the Mac App Store’s purchasing system is not designed so that a podcasting company can spend $15 for GarageBand and equip 30 different Macs with a copy. But if you want to buy a game and put it on your desktop Mac, your laptop, your wife’s laptop, and your daughter’s laptop, go for it.