A sea knoll is a small seamount. Specifically, a seamount is any underwater mountain that rises over 1000 meters above the seafloor, while a sea knoll is anything from 500 to 1000 meters. Mounds smaller than 500 meters are sometimes lumped in with knolls, but are properly referred to as abyssal hills. The most common cause for all of these are underwater volcanoes both active and expired.
As higher-tech scanning methods allow for better maps of what exactly lies below the waves, these terms grow more precise and more useful. Knolls account for 16% of the worldwide seafloor (while seamounts are just under 5%), making this type of landscape an important influence on marine ecologies. While there is variance, knolls tend to provide a rocky substrate and differences in current as the water moves around the knoll (high current means more food coming by). They also give bottom dwellers a chance to be a bit nearer the sunlight, although this effect is only meaningful in the shallower deeps.