Over 90 different species.
Krill look like a small shrimp. Ironically, this is their distinguishing characteristic. If you are a serious biologist, you might note that shrimp have ten legs, while krill have 12 to 16.
Krill live in every ocean on Earth, and have a cumulative biomass in the billions of billions of tons. Just about everything bigger than a krill eat krill; many species of whales, seals, penguins, squid, and fish depend on krill. Krill, in turn, are filter feeders that eat phytoplankton and zooplankton. They range from 10 to 150 millimeters, and thus some of them are indeed big enough for humans to catch with nets. If humans do go after krill, they must freeze them immediately, as krill turn poisonous at room temperature.
Krill live in in the photic zone, although many travel deeper during the day to escape predators. Many species take part in the grand diurnal vertical migration, traveling to the surface at night to hunt plankton. Other species will swim towards the surface whenever they are hungry, and may make the trip to the surface multiple times a day. Krill travel in large swarms to protect themselves from smaller predators. Of course, a large swarm of krill can also attract larger predators, and swarms can disband at the drop of a hat. Some fleeing krill have been seen molting to leave behind exoskeleton fragments as decoys.
Most species of krill have light-making organs, an array of ten of them in lines along each side. These photophores are comparatively complex, with lenses allowing them to be 'focused', and with the necessary musculature to allow the krill to rotate them. We do not know what these are used for, although it is generally assumed that they either have a social/sexual function, or that they are some form of camouflage.
The word comes from the Norwegian word krill, meaning 'young fry'. A group of krill is known as a swarm.