Locusts are a species of short-horned grasshopper (named after the short, stumpy antennae). You may know them from the chirping sounds they make during the day. They do this by rubbing the spines on the hind legs against their serrated wings.
They are solitary most of the time, however, if they are doing particularly well, food is abundant, and the environment becomes over-crowded, swarming behaviour is triggered. This is when it can get bad for farmers. A swarm of locust can deplete a field of crops in a matter of days, and they're challenging to deter once they've begun swarming.