It's a nocturnal creature, so you've probably only seen it--or one of its many cousins--scurry for cover when you overturn a log or rock in the garden.
A gardener's friend, the predacious ground beetle feeds on caterpillars, cutworms, slugs, tent caterpillars, fly maggots and other garden and household pests, crushing them with powerful chewing mouthparts. Although it's has been known to feed on earthworms and a few other "good" bugs in the garden, the predacious ground beetle eats far more bad bugs than good.
Sometimes called carabids (they belong to the Carabidae family), predacious ground beetles are usually black or very dark brown, and sometimes metallic green or purple. Their head is usually considerably smaller than their body. They're often found in moist to wet locations. Some types exhibit defensive behavior (like emitting strong odors) when disturbed. They're fast runners.
These beetles are sometimes attracted to lights and find their way inside the house. If you find one indoors, carry it outside and release it. It's a good guy.