Caterpillars are the larvae from butterflies and moths.
They look like little worms with 6 legs near the head and some additional stumps at the other end. Caterpillars have a wide range of colours, usually those that allow them to camouflage, browns, yellows and greens prevail. They come with smooth skin, covered in bristly hairs and/or warty tubercles. They often appear to be bird droppings.
Caterpillars are voracious eaters and are usually specialised to feed on a particular plant family. They can be serious pests eating away leafy plants in no time, or infesting every fruit of a tomato plant.
Look out for the actual animal when inspecting plants. Leafy greens affected by caterpillars often show serious leaf damage. You will often find caterpillar droppings on infested plants. Some caterpillars produce silky threads like spider webs. Infected fruit often show holes.
There are several ways to prevent caterpillar damage and to deal with infestations, read more here.