Silencers reduce the sound of the shot from your weapon, or hide a longer innerbarel.
Tracer Units often come in the shape of silencers. Through an internal flash-unit, they illuminate glow-in-the-dark/tracer BB's. This allows you to see the trajectory of the BB's in the night/dark. This simulates tracer munition.
Flash Hiders are mostly esthetic, and are used to customise the look of your replica. They come in different shapes and sizes. Some have features like QD Silencer compatibility or sound amplification.