Nina holding a katydid, Pitilla Biological Station, Costa Rica (Photo RGT)
Bugs are insects, and the word "bug" is a common name for any insect. But what exactly is an insect? Insects are animals with six legs, or three pairs of legs. Insects are arthropods; their skeleton (called an exoskeleton) is on the outside of their body. The exoskeleton covers the entire outer surface of the animal. The muscles are connected to the inner surface of the exoskeleton. The exoskeleton consists of a series of hard plates connected by membranous or soft tissue that acts like a joint, so that the animal can move around. Insects have a segmented body with three body regions: the head, thorax and abdomen. They have one pair of antennae (pronounced an-ten-ee; singular antenna) on the head. The antennae are the primary olfactory organs; that is, they are the insect equivalent of our nose. Most adult insects have two pairs of wings.
You may have heard the term "true bug". True bug, or simply bug, is also the common name of a specific kind of insect. There are many families of true bugs; for example, stink bugs (family Pentatomidae), assassin bugs (family Reduviidae), and plant bugs (family Miridae).
Finally, "bug" can also mean any small animal classified as an arthropod, such as a spider, sowbug, or a millipede. All arthropods have an exoskeleton, but different arthropods have different body plans. Spiders for example are arthropods but they are not insects. Spiders have four pairs of legs as opposed to three, and two body regions instead of three. Millipedes are another kind of non-insect arthropod. To read a wonderful introduction to the arthropods, go to the "Arthropod Story" at http://www.evolution.berkeley.edu/evolibrary/article/arthropodstory