It's easy to recognize larger pests, such as deer and rabbits--but it's not so simple with the smaller ones. They're often too tiny or too fast-moving to be easily seen, and they tend to conceal themselves well. Sometimes they look like their beneficial relatives: most of us can't tell the difference between a spined soldier bug (helpful) and a squash bug (hateful). Sometimes they manage to let other creatures take the rap. Pillbugs, for example, are often wrongly convicted through circumstantial evidence: we find them at the scene of the crime and assume they did the damage. And some pests confuse us because they change their appearance as they mature: one day they're grubs living in the ground, the next they're shiny beetles scampering among the plants.
Note: For convenience, gardeners typically refer to most pests simply as "insects." But while many garden pests are in fact true insects--aphids, grasshoppers, and squash bugs, for example--others are only insectlike. As far as the gardener is concerned, they're related to true insects in the harm they do; but they belong to different scientific classes. Examples of such creatures include spider mites (arachnids), nematodes (roundworms), and slugs and snails (mollusks).