A launchers is a program/app that helps you find and open programs/apps with ease. There are different types, for example, Android Launchers (aka home-screens), Windows launchers, OSX launchers, Linux launchers (places for program shortcuts)
An Application Launcher is a computer program that helps a user to locate and start other computer programs. It basically exists on every OS where the user interacts with the computing device – be it desktop, smartphone, or tablet. A comparison list of desktop launchers can be found on the linked Wikipedia page.
On Android, the launcher is also often called "Home-Screen", as pressing the "home button" brings the user here. Android launchers are compared on this Wikipedia page, to give you a raw overview. It's probably impossible to keep a complete list up-to-date, so don't expect this being one.
When should this tag be used?
Of course when you're looking for a launcher – and not in any other case.