Paper 2D in Unreal Engine 4 (UE4) is a sprite-based system for creating 2D and 2D/3D hybrid games entirely within the editor. At the core of Paper 2D are Sprites (which are a Texture Mapped Planar Mesh and associated Material). You can edit Sprites inside UE4 with the Sprite Editor and create sprite-based animations with Flipbooks (which animate a series of Sprites sequentially by using key frames and specifying a duration in frames to display them).
Refer to each section below for more information and the Paper 2D map inside the Content Examples project for additional examples: