Periodical Cicadas are large, chunky insects with bulging eyes on the sides of their heads. They are slow fliers and easy to catch by birds and other flying insects (i.e. wasps). Their abundance in the years their population explodes means an easy feast for their predators. They have no real defense against predators except large numbers - the neighboring cicada may be more appetizing.
Though Cicadas are generally found in forested areas and grasslands, the species can become a common sight (and sound) in neighborhoods during the summer. Folks from the Mississippi River basin to the eastern United States and Canada are pretty familiar with the Periodical Cicada. While they are completely harmless, large numbers of them can become a nuisance to some people, covering cars and driveways, making it 'uncomfortable' to walk outside or drive on the unwitting insects (brings to mind the buggy cave scene in "Indiana Jones: Temple of Doom").
Females will insert eggs into a tree branch or bark and the hatched nymphs eventually make their way down the tree into the ground. They burrow down by the tree's root system. Total maturity time of the nymph is about 13 to 17 years (hence 'periodical'). They eventually resurface after almost 2 decades underground and begin molting its early exoskeleton. There are always a few that develop a year or two before or after the masses, meaning you are likely to see at least a handful in 'off' years.