You can have it either way. You can have a copy of your desktop on the second monitor (I assume to make it easy for two people to see the same thing if they're in different locations) or you can have the second monitor as an extension of your first monitor. A window can be dragged from one monitor to the other just like your mouse jumps from one monitor to the other. The taskbar and start menu and clock, etc will only be on the primary display.

If you maximize a window it will only maximize to the size of one monitor. It is slightly more appealing if both monitors are the same size and have the same resolution because the secondary monitor will have wallpaper that is exactly the same as the primary monitor. If the second monitor is larger or smaller than the secondary monitor then the wallpaper will not look the same on the second monitor. Other than that, it all works fine if you have different sizes of monitors.