Filter foam, pads, and other materials mechanically filter large particulates (ie: left over fish food and fish debris) from the aquarium before it reaches further into the filter. They are generally used as pre-filter material in the first stage of aquarium filtration and should be cleaned/replaced frequently to prevent them from clogging and prevent the filter from depending on the growth of nitrifying bacteria on their surfaces.
Chemical Filtration is the 2nd stage of the filtering process using products such as activated carbon and other chemical-absorbing materials that get rid of chlorine, chloramine, ammonia, copper, medications, phosphates, odors, and dyes. They too should be changed (or re-charged) frequently.
Biological Filtration is the last stage of filtering. Bio Balls, Bio Stars, Ceramic Rings and other "bio" materials provide a surface area for nitrifying bacteria to colonize and grow. The bacteria eat the remains of toxin-producing biological activity and finally return the water to the tank clean. Do NOT sterilize or clean these materials unless absolutely necessary. It will strip the tank of bacterial protection. Read More About Aquarium Filtration > >