Niagara is the new particle effect system in Unreal Engine, and it will eventually replace Cascade. It is still partly experimental or beta, and you have to specifically activate Niagara plugin, if you want to use it. It is radically different from Cascade, and best way to start experimenting with it, is to read the appropriate post in
which will explain the main features. Also, you can experiment with plethora of samples in
Just download it and try it out.
Unreal Engine Pathways 