Genymotion is an emulator for emulating Android on your PC, with the aim of being faster than the Android emulator that comes with the Android SDK. It's free for basic use, with tiers of paid license for extra features including fake sensor input.
Related tags:-
- android-emulator for emulating Android in general, or questions about the emulator that comes with the Android SDK
- virtualbox, a VM often used to emulate an x86 Android box
- emulator for questions about emulating other systems on an Android machine (commonly old games consoles)