The nymph of the two-lined spittle bug is noticable as what appears to be a drop of spit on a plant, but in actuallity is substance created by the insect to shield it from preadators. This spittle like appearance is where the insect gets part of its common name. The adult (pictured) is a black leaf hopper with orange stripes, hence the name two-lined spittle bug.
While the nymph causes very little damage to plants from feeding, the adult can do significant damage to the asthetic appeal of the plant (notice the damage to leaves in the picture). Generally speaking, this damage has little affect on overall plant health, and treatment is usually not warranted.