What is the "Launcher" application and why are there alternative apps? Several questions have mentioned launcher.  What is it responsible for, and what kinds of features do the alternative apps in the Market provide that the basic android launcher doesn't?
Launcher is a replacement for the Home application (the app that your phone switches to when you press the Home button).  It is basically the application that lets you switch screens, launch apps, view widgets, etc.  Other apps offer more home screens than the default, or different toolbars at the bottom of the screen offering different ways to quickly launch applications.  These were very popular with the initial Moto Droid since it was basically the stock Android 2.0 OS which only had 3 home screens.  With Android 2.2 and the custom interfaces that handset manufacturers add on, the usefulness of these launcher apps may not be so obvious or necessary.