A Launcher can be compared to your computer's desktop: this is what you get to see after starting up your Android device and unlocking the screen. You can customize it (changing the background image, placing home-screen-shortcuts and widgets are among the core functionalities all launchers provide). An synonym often used for "launcher" is "home-screen".
Most launchers also include the app-drawer – i.e. the place where you can find (and launch) all installed apps, and usually also their widgets.
Replacing the pre-installed launcher
Android naturally comes with a launcher being pre-installed. Depending on your device and ROM, this might be one of many:
- Samsung devices have their own TouchWiz-based launcher
- LG has its own framework as well (LG Optimus UI)
- same to be said for Motorola devices coming with Motoblur, HTC Sense, Asus Zen UI, the Xolo Hive UI, etc.pp.
- CyanogenMod (and some other AOSP based ROMS – see aosp-rom) use the Trebuchet launcher
And so on. You can always "replace" the pre-installed launcher by simply installing another one (e.g. from Google Play Store). Having done that, the next time you press your Home button Android will ask you which one to use – and you can make your selection permanent.
When you replace one of the launchers coming with a specific "manufacturer's framework" (as named above), you might lose some of the framework's functionalities: widgets designed especially for that (e.g. LG's weather widget) won't work with your "replacement". No danger with that, though: if you want to "revert back", simply remove your replacement – or use a Default Homescreen Switcher to change the default (which can also be done by going to Settings › Apps, scrolling to your replacement, open its entry, and reset the defaults here – which then lets you select your launcher again on the next press of your Home key).
- use app-drawer when your question is explicitely and dedicatedly relating to the drawer only, but not to the home-screen component
- same way, just opposite, use the home-screen tag when not relating to the app-drawer
- use launcher when either referring to both, or not being sure which part you're referring to
However, these tags should be used in XOR manner: no need for any combination ;)
- List of Android Launchers at Wikipedia
- List of Android Launchers at IzzyOnDroid
- Everything you need to know about Android launchers at CNet
- Android A to Z: What is a launcher? at AndroidCentral