Most of us often have many Mac apps open at the same time, flicking from one to the other as we need to do different things. In reality, many of us don't even close our apps once we're done with them. We leave them open for quick access should be need them again. There's no real problem with doing this nowadays, since the hardware is generally fast and memory is readily available.
There are various ways to switch from one app to another, like clicking one of the windows belonging to that app, using exposé, clicking the app's icon in the dock and so on. However, there is another way. Many people probably don't know about this one and it's really, really useful.
At any time, you can press and hold the CMD key and while still holding the CMD key, simultaneously press the TAB key once. You will see a new menu appear in the middle of the screen, which looks a bit like the dock, but this menu shows you all the open apps. As long as you keep pressing the CMD key, this menu will stay on-screen.
To move from one app to the next, just keep tapping the TAB key. When you get to the app you want, release the CMD key and that app will come to the front.
This feature also has another nice benefit, which is that you can actually issue commands to the apps while using CMD-TAB. For example, if you want to quit various apps quickly, you can hold the CMD key and press the TAB key until you reach the first app that you want to close. While still holding the CMD key, simple press the "Q" key. This will ask the currently selected app to quit. You can then, while still holding the CMD key, use the TAB key to move to the next app. Again, press "Q". If you repeat this a few times, you will see what a powerful way it is to close multiple apps in just a few keystrokes!