Android launcher is nothing but the user interface you see in the home screen. You might have noticed the interface between Samsung and Sony differs. It is because Samsung use Touchwiz launcher and Sony use Xperia launcher. Now you might think that launcher is similar to a theme. We cannot say launcher is a theme because launcher changes the app drawer. For example, in some launcher you can use gesture to launch an app. Theme will just apply a wallpaper and changes icon to an existing launcher. Most of the Android phone runs stock Android launcher, it is default launcher. There are many launcher in Play store. Some launcher will utilize less RAM memory, so you can install it in low end device to make it faster.
Different Android launcher:
There are many launcher in Play store available for free. However few of them charge you. Some of the popular Android launcher are Nova Launcher, Apex, Google Now, Buzz Launcher, and Nokia Z launcher.