The Niagara VFX System is one of two tools you can use to create and adjust visual effects (VFX) inside Unreal Engine 4 (UE4). Before Niagara, the primary way to create and edit visual effects in UE4 was to use Cascade. While Niagara has many of the same of particle manipulation methods that Cascade offers, the way you interact and build visual effects with Niagara is vastly different.
This page links to documentation regarding the Niagara VFX system, including an Overview and a Quick Start guide to get you up and running. If you are new to Niagara, it is recommended that you check out the Getting Started section which includes a high-level overview of the Niagara Editor, the Quick Start Guide, and a page that lays out the Key Concepts and design philosophy behind Niagara. You can also check out the Guides section, which has How-To articles on various tasks. The Reference section has the System and Emitter Module Reference, the Niagara Editor UI Reference, and the Niagara Script Editor UI Reference.