This is the most fundamental question. Think of them as performing two different acts in a three-act play about clean water.
Multimedia Filter (MMF): The Physical Debris Remover. An MMF is a depth filter designed to remove suspended solids and particulate matter from water. It's called "multimedia" because the tank contains layers of different materials with varying densities—typically anthracite coal on top, sand in the middle, and garnet or gravel at the bottom. As water flows through, larger particles are trapped in the top layers and smaller particles are caught deeper down. Its primary job is to reduce turbidity (cloudiness) and remove things like dirt, silt, rust, and sediment. It's a physical separation process.