Poison oak is most common in California, western Oregon, and western Washington. In the open or in filtered sun, it forms a dense, leafy shrub; in the shade, it's a tall-growing vine. Its leaves are divided into three leaflets with scalloped, toothed, or lobed edges. Poison ivy is quite similar in appearance; it's common east of the Rockies and also grows in eastern Oregon and eastern Washington. Usually found in shady areas and at the edges of woodlands, it sprawls along the ground until it finds something to climb; then it becomes a vine.
A resin on the leaves, stems, fruits, and roots of both poison oak and poison ivy causes severe contact dermatitis in most people. Both these plants are spread by birds, who eat the fruits and disperse the seeds. Poison oak and poison ivy are most effectively controlled with an appropriately labeled herbicide, such as triclopyr or glyphosate (take care to avoid getting these chemicals on desirable plants).