Trilobites belong to Arthropodia, the phylum of creatures with jointed legs such as spiders, insects, shrimp, and their relative, the modern horseshoe crab, which trilobites resemble. Their name, three lobed, refers to the plainly visible division of their bodies into the center axis -- called an axial lobe -- and the pleural lobes to either side. Most of them were not large, though some reached 70 centimeters (a little over two feet) in length.
Physically, trilobites are notable for two innovations. The first one is their carapace, which incorporated the mineral calcium carbonate to give it strength. This resulted in the growing trilobite having to molt, or shed its carapace, many times during its life. The resulting cast-off shells are part of the fossil record along with the remains of the creatures themselves. The second innovation -- and possibly the most crucial from a human perspective -- is that they are the first life form to develop eyes. While these were compound eyes, very similar in form to the ones insects have today, ultimately they led to the binocular vision which is our primary way of perceiving the world.
Trilobites reached the apex of their development by the end of the Cambrian Period, about 500 million years ago, but they were abundant through the end of the Permian Period (230 million years ago), when about 90 to 95% of marine species became extinct.
Information for this article came, in part, from Trilobites, by Riccardo Levi-Setti.