What is the difference between sealer and filler?

Sealer has the meaning of "adhering, covering". It is an undercoating material that is mainly applied to absorbent substrates such as mortar and concrete. It acts like an adhesive to enhance adhesion to the substrate. On the other hand, filler has the meaning of "filling, stuffing, filler". It is mainly used to repair cracks and fill uneven surface to make it smooth.