Late to the game, Microsoft is adding a store to Windows, much like the marketplaces for Mac OS X, iOS, and Android. If you want to buy apps from the Microsoft Store, you need to create a Microsoft account.
Perhaps I should say stores, since you'll find more than one store within Windows 8. You buy Windows 8 apps by clicking the Store tile--but you purchase music by launching the Music app, and you buy videos by launching the Video app.
Even more confusing, the app store is called just the "Store" while the music and video stores are named Xbox Music and Xbox Video. (Of course, both the Music app and the Video app are media playback tools as well, though they are less robust compared with Windows Media Player or the likes of iTunes. The new operating system's lack of a unified Windows 8--style media player is a pretty significant hole.)
Navigating the Microsoft Store is similar to navigating the Start screen. Featured apps come in individual tiles, and are sorted by groups; each group also has a 'Top Free' tile and a 'New releases' tile. As of this writing, however, the Store listed only about 1000 apps, so Microsoft has a little catching up to do. The number of apps available at the official Windows 8 launch on October 26 will be more telling.
If you don't like Windows 8 out of the box, you can customize it, with some exceptions. Perhaps the most controversial exception (as mentioned earlier) is the fact that you can't set Windows to boot directly to the desktop, though third-party utilities promise to enable this.
Since the Start screen consists of groups of tiles, moving your favorite or most commonly used tiles to the left side of the screen is pretty easy. You can also specify the tile size (normal or double-wide) and turn off live-tile updates if you find them distracting. In addition, you can group tiles by program type, such as business applications, games, and so on.