The name tri-lob-ite refers to the three lobes that are seen in all members of this group. Each trilobite has a long "speed bump" running down the center of the body (the axial lobe) and one lobe on either side of the axial lobe (the pleural lobes).

The cephalon in trilobites commonly has a pair of compound eyes (as in the specimen on the left), although not all trilobites had eyes. The specimen on the left also has a pair of short genal spines pointing backwards from the lateral edges of the back of the cephalon. Genal spines can be quite elaborate in some groups, while in others genal spines were absent. Many trilobites had pronounced facial sutures, which cracked during the shedding of the exoskeleton (ecdysis), and allowed the animal to escape from its former integument. While they are not usually preserved, some trilobites also bore a pair of antennas on the cephalon.

The thorax in trilobites is composed of between 2 and several dozen individual thoracic segments When the animal was alive, each of these segments bore a biramous walking limb. Thoracic segments are by far the most common fossil trilobite remains in most deposits.