A sump pump is a small pump installed in the lowest part of a basement or crawlspace. Its job is to help keep the area under the building dry and to prevent it from flooding. Usually, sump pumps are installed in specially constructed sump pits. Water flows into the sump pit through drains or by natural water migration through the soil. The sump pump's job is to pump the water out of the pit and away from the building so the basement or crawlspace stays dry.