A trilobite is a type of fossil. Trilobites were arthropods — small, segmented animals with exoskeletons — that lived in Paleozoic times.
Trilobites were marine animals with many legs, their bodies divided into segments (like spiders, scorpions, and caterpillars). The back of a trilobite's body had three sections, or lobes. The word trilobite, in fact, means "three lobes" in Greek, from tri and lobos. We know a lot about trilobites, especially considering they've been extinct for a long time, because their exoskeletons were easily fossilized.