Back in the historic times of Mac OS pre Mac OS X you had an Application menu up in the righthand corner of your screen that you could use to quickly change between open applications. Pre-Mac OS 9, in certain system versions, there was something called the Switcher that you could use for the same thing. In Mac OS X, up till and including Jaguar, you hade to make do with the Dock for applications switching (nothing wrong with that, it works very well).
From and including Mac OS X 10.3.x Panther you got an onscreen switcher for switching between open application.
To get the onscreen switcher, just press + Tab.
On the screen you get a huge banner with the icons for your open applications.
If you keep holding down the and press Tab, it walks you through the icons.
When you stop on an icon, that application is brought to the front. The banner over the screen looks something like this: