You can identify wasps and hornets by examining the insect's size, markings and colors. Other clues to help differentiate wasps from hornets include the insects' nesting behaviors, living situations and temperaments.
Wasps are typically slightly larger and have narrower bodies than either hornets or yellow jackets. Another identifying characteristic of many wasps is their narrow waists, which most hornets lack. Unfortunately, color is very helpful in identifying wasps, as there are many different color variants and combinations.
Wasps have two main nesting habits: solitary and social. Solitary wasps - like ichneumon wasps, spider wasps and mud daubers - usually dig a burrow where a single female lays her eggs. These nests are very small and not easily seen. Social wasps form large nests made of a fibrous material, giving some species the name "paper wasps," though yellow jackets, European hornets and other species make their nests from the same material.
Hornets and yellow jackets have smaller, thicker bodies than wasps. Hornets are either black with yellow, black with brown or black with white. Yellow jackets are exclusively black with yellow. Both hornets and yellow jackets are exclusively social, forming large nests of fibrous material. Unlike wasps, which are often docile, yellow jackets and hornets are nearly always territorial and aggressive.