The redwoods in Northern California (two species in different genus but the same family) are commonly called "Coast Redwood" for Sequoia sempervirens and "Giant Sequoia" for the Sequoiadendron giganteum. As a family of related trees (there are only three species in the world) they are called collectively either "redwoods" OR "sequoias". At one time, redwoods domininated the world’s temperate forests, and what you see are relics from a different age that somehow survived in pockets here and there.

You will need to fly or drive to Northern California to see the trees. San Francisco is the entry point to the Redwood Empire, primarily in forest pockets to the north of the city, along the coast to the Oregon border. They don’t grow everywhere and conditions have to be PERFECT for them to grow and thrive; primarily the main factor is sufficient moisture during California’s dry summers from the fog that rolls into Northern California from May to September. There ARE groves of redwoods south to Monterey and Big Sur however, but the most beautiful are north of SF in the Avenue of Giants along highway 101. These are the tallest trees in the world, and some trees are thousands of years old. This is about 3-4 hours north of the city however; so if you want something closer just to see what a redwood forest is like, you can visit Muir Woods National Monument, about 30-45 minutes north of San Francisco in Marin County. Muir Woods is beautiful, but except for a few groves is largely secondary growth, i.e., it was close to town and the forest was originally chopped down for lumber…what you see is regrowth not original forest. So go north to Humbolt County if you want to see the tallest and oldest trees. The trees are so tall and straight, they block much of the light to the forest floor where only sword ferns and low shrubs florish; the effect is like a natural cathedral made of trees.
http://www.terragalleria.com/california/california.avenue-giants.html

Giant sequoias are sadly a lot rarer than coast redwood forests, and only exist in patches here and there in the Sierra Nevada mountains, where by pure chance they survived. The Sierra Nevada is the range of mountains that separates California from Nevada You can conveniently visit some groves of giant sequoias in Yosemite National Park, at the south gate to the park. There is also a national park south of Yosemite dedicated to these trees, but unless you have lots of time, it is very difficult to get to and from this park. Giant sequoias are MUCH larger (but shorter) than coast redwoods, and are the largest trees in the world.
http://www.yosemite-tours.com/giant_sequoia_tours_in_yosemite.htm

Redwood is a general name for trees in the family Taxoidaceae. It includes the Dawn Redwood (Metasequoia glyptostroboides). The tree you are keen to see is the Coastal Redwood, Sequoia sempervirens. The closely related Sequoia giganteum (once called Sequoia wellingtonia) is also found in California but tends to be slightly shorter.

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