Rotifers are tiny shrimp like creatures who have their own phylum, rotifera. Their mouths are like "wheels with teeth" so they are nick named "wheeled animals" they dont have an anus but do have a body cavity. They feed mostly on live plants, including mainly phytoplankton.
People breed them mainly to feed larval fish and to help increase the amount of live food available to corals.
Newly hatched baby brine shrimp is a better choice to feed live food to a reef tank. Rotifers take a great deal of patience and hard work to cultivate in any numbers large enough to benefit a large system.