Game development,… i really have no simple definition for this one; but it involves developing mobile, browser, tv, computer and other devices games using 2d graphics (images), 3d models and some programming. This field go hand in hands with graphics design and 3d modeling & animation and hence divided into two: 2D and 3D game development. Though the two are not really much different. There are a lot of way to develop games and many technologies to use.
2D games: In developing 2Dgames, one needs 2D graphics (often called sprites) and some coding. In this case, i prefer to use Unity over any other technology/game engine.
3D games: In developing 3D games one needs to have beautifully rigged and sometimes animated 3D models and some programming knowledge. Although i would chose Unity over any other game engine, my recent exploration with UnReal Engine made me realize that i had a choice on how to go about developing 3d games; Unity or UnReal.