Redwoods have an interesting taxonomic history. Although several species of redwood (Sequoia) once spread across the globe, long-term climate changes have reduced their numbers and their range. Now, only one species exists, Sequoia sempervirens, and it occupies a narrow band along the west coast of North America, from southwestern Oregon to Monterey, California.
Two other trees are commonly confused with redwoods, but each is a separate genus: giant sequoia, Sequoiadendron, and dawn redwood, Metasequoia. Prior to the formation of the Cascades, when the Pacific Northwest's climate was warmer and wetter, all three "redwoods" grew here. Now, giant sequoia grows naturally only in California, while dawn redwood is native to China. As with redwood, each of these trees has been widely planted outside its native range.
For more information on the redwood native to the Pacific Northwest, go to the species page or see p. 40 of "Trees to Know in Oregon".