Cascade is the old particle system tool in Unreal Engine. It is still great, but comparatively is simpler and has more limitations than Niagara. Niagara is similar to Cascade and familiarity with Cascade will help familiarize you with Niagara. However, Niagara permits you to expose and modify variables in the engine more directly, gain access to more engine-level data, and as a result create more "complex" effects.
A quick overview of Cascade's UI is below along with one for Niagara Emitters and Niagara Systems for comparison.
Cascade Emitters + System
Niagara Emitter
Niagara System
Enable Niagara (pre Unreal Engine 4.25.0) through Plugins → FX → Niagara. Niagara should be enabled by default in engine versions 4.25.0 and above.
