How does buying an iPhone from an apple store work?

If you buy an iPhone from the apple store do you just buy the phone itself and then have to take it to you phone carriers store to get it activated? Or?

IPhone 5s and iPhone 5c are now available for walk-in purchase on a first-come, first-served basis at your favorite Apple Retail Store. When you come in, we'll help you pick the right rate plan, transfer your contacts, set up your email, and more.