By selecting early sasanqua through late japonica varieties, camellias bloom from early fall through late spring in Southern gardens.
(Editor's Note: This article was originally published on February 12, 2008. Your comments are welcome,but please be aware that authors of previously published articles may not be able to respond to your questions.)
During the late 1960s, I had to be at the San Diego Zoo several times week for a class with Albert, the silverback gorilla.
One day after class I decided to explore the rest of Balboa Park. I wound up in an overgrown and neglected Japanese garden. It was the first time I had come face-to-face with hundreds of camellias all blooming at the same time. Later, I learned that the "secret garden" I had stumbled onto was the ruins of the Japanese tea garden designed and built for the 1915 Panama-California Exposition in San Diego.  When I found the garden, it had been left to its own devices. Blooming plants were sprawling and climbing up the remains of ancient crumbling masonry walls, all in camellia colors: white, red, pink.
When I moved to Greensboro, Alabama I learned of more untended ancient camellias, still blooming away. The local legend was that these camellias were planted on the grounds of a house built before the Civil War as a wedding present to a daughter. The camellias were air-layered slips from originals planted in the 1830s on the adjacent property of the bride's parents. This property is still known as “Japonica Path“ for its camellias. When I first visited this nineteenth century bride’s abandoned and neglected garden, the camellias were flourishing as if time were of no consequence.
In their native habitats in Asia. China, Korea, Viet Nam and Japan, camellias grow on well-drained hillsides under the dappled shady canopy of tall trees. While some tolerate sun more than others, most prefer a replication of their native situation, planted under tall trees, in acid humus-rich well drained soil. And for me they always look best the way I first saw them, planted in an understory forest in large groups with other shade loving plants. However, the tall sasanquas make wonderful informal hedges, and some of the exquisite flowered japonicas deserve to be featured garden plants.
Camellia breeders in both Japan and the United States are working to produce plants with more cold hardiness. In the United States camellias are normally hardy through Zone 7b. There is a collection in Washington D.C. at the National Arboretum.  The plants will however be damaged by occasional hard freezes. Camelliagirl discusses frost protection for camellias, Camellia sinensis, tea camellias, and growing camellias from seed in this thread.
In Asia there are many species of camellias. Camellia sinensis, the tea camellia, is grown on tea plantations for the commercial production of tea. (Photographs 2 and 3). Camellia oleiflera is usedfor the production of camellia oil from seed pods [Photograph 4]. Camellia oil is used in cosmetics because the oil is similar to human skin oil . It is used as hair oil by the Sumo wrestlers of Japan. During the Japanese Edo period (1603-1867) Camellia oil was used to prevent corrosion on Samari swords. This tradition is still maintained by Japanese woodworkers to prevent corrosion of metal tools 
And, of course camellia seeds can be used to grow more camellias. Seed grown camellias, though, are not likely to resemble their parent plants. For this reason, most home growers prefer to propagate named camellias from hardwood cuttings or air layers. 
While there are many camellia species in Asia,  in the United States only two ornamental species and the hybrids between them are commonly available, Camellia japonica and Camellia sasanqua. Both of these species are indigenous to Japan. The spectacular late-blooming Camellia hiemalis is possibily a sasanqua-japonica hybrid. The tender Camellia reticulata is not widely available. The limited availability of camellia species in this country is largely because the native habitats of Camellias in China, Korea, and Viet Nam have been politically inaccessible to American plant breeders.
Sasanqua camellias begin blooming in October and their flowers are often large single blooms. The Japonica camellias tend to be not so tall and their flowers are more formal in presentation.
There are a few single flowered Camellia japonica cultivars, but most are double flowered. The American Camellia society classifies camellias on the basis of flower form. Camellia Nomenclature is a biannual serial publication of the American Camellia Society that reviews and defines the classification of camellias. Currently, six classes of camellia are defined on the basis of flower form: Class I. Single. Class II. Semi-Double. Class III. Anemone. Class IV. Peony. Class V. Rose Form. Class VI. Formal Double. Roseform and Formal Double flower forms are characterized by imbricated petals -- the petals are layered like fish scales.  The cultivar "Pink Perfection" is the classic example of the formal double imbricated flower form and there is at least one plant in nearly every Southern garden. "Pink Perfection" is featured in this thread by GenePhillips.
There is also a group of miniature flowered camellias. (See Photograph 12, Tiny Star).
Traditionally camellia colors in the United States are white, pink, and red, but in the 1970s China opened its doors to botanists from the West. Seeds for yellow camellias (Camellia nitidissima) became available to western breeders. The chinese yellow cultivars were off white and pale yellow. "Buttermint", "Ki-No-Muto No. 95" are both soft yellow flowered camellias, while "Brushfields Yellow" is off white and "Golden Glow" is pale yellow.
A few years later yellow Vietnamese camellias became available.  These were camellias with stronger yellow colors.
There are now about 25 species of yellow camellias contributing to hybridizing programs in the United States and Japan. 
To learn more about camellias, I would highly recommend monocromatico's web bibliography of links to camellia websites in this thread.
As a result of the lifting of the iron curtain the full genetic diversity of camellias is now available to plant breeders. New, more practical, more cold hardy, and even more beautiful camellias should soon be available at your local nursery.
14. Camellia chrysantha 15. Infant Gorilla
 The Tea House. 1915 Panama-California Exposition, Balboa Park, San Diego. (Postcard Photo) www.sandiegohistory.org
 William A. Mc Namara. Illicium simonsii. Quarryhill Botanical Garden 2001-2005. www.quarryhillbg.org
I am a retired archeologist and curator of an historic house museum. I live in Greensboro, Alabama, a small rural historic Southern town, with my two dogs, a rabbit and (by recent count) two cats. I am upgrading a 100 year old neoclassic house and clearing and planting my two-and-one-half acre property. Of plants, I love roses best of all.