insect v. bug

A giant water bug. Image courtesy of The Ranger’s Blog. True bugs are insects belonging to the order Hemiptera, such as aphids, leafhoppers, shield bugs, bed bugs, and cicadas. Hemipterans are characterized by the presence of elongated mouthparts used for piercing tissue and sucking fluids. In addition, their forewings are generally thick at the base and membranous at the tip, and they typically have five-segmented antennae and three-segmented tarsi.

Colloquially, bug can refer to most terrestrial arthropods (e.g., other insects, spiders, centipedes) or to bacteria.

BugGuide.net on Hemiptera