Trilobites were one of the first animals on Earth. They are a type of arthropod and share a common ancestor with insects, spiders and crustaceans.
Trilobites weren't very biodegradable and they litter up the fossil record with a preservation bias. Like all arthropods, Trilobites had an exoskeleton, which they shed as they grew. These exoskeletons were made of calcite and preserved very well. Trilobites could shed their exoskeletons up to 7 times. So each individual trilobite had up to 7 chances of being preserved.
Trilobites evolved during the Cambrian, and diversified and proliferated during several of the following geological eras. There are multiple families and sub-orders of trilobites -- like pokemon you have to catch them all!