All true tea comes from the plant Camillia sinensis. Herbal "tea" should actually be called tisane (ti-'szan) because it does not come from this plant. Camillia sinensis is grown in certain regions of the world: China (Keemun, Yunnan, Fujian), India (Assam, Darjeeling, Ceylon, Nilgiri), and Japan.
 
Tea is processed in a number of different ways. How the tea is processed will affect its' flavor and caffeine content:

 

There are three main types of tea. They vary according to how they are processed. Green tea is withered slightly and immediately fired. There is no oxidation. Oolong tea is withered, rolled and moderately oxidized before firing to stop oxidation. Black tea is withered, rolled and "fully" oxidized then fired.

 

The amount of caffeine in the tea depends upon the plant variety, stages of growth and parts of the plant used. The longer you brew the tea the higher the amount of caffeine. Compared to coffee, green tea has about 10%. Oolong has about 25%, and black tea about 50-65% of the amount of caffeine per serving. If you are transitioning off of coffee this information could be useful to you. You might choose to transition from coffee to a black tea first then move toward a milder green tea over time.
 
The polyphenols in tea are thought to provide the health benefits. There are about 30 of them in tea. There is a product called Polyphenon E that has been approved by the FDA for the treatment of genital warts (Meltzer et al., 2009, American Journal of Obstetrics and Gynecology) and now it is being studied for prostate cancer. A recent review of the literature on green tea concludes that overall clinical evidence is inconclusive (Clement, Y., 2009, Preventive Medicine). The authors also state that it looks as though habitual tea consumption may be chemopreventive against breast and prostate cancer. It may also attenuate risk factors for atherosclerosis decreasing the incidence of cardiovascular events and stroke. These conclusions are based on both interventional and observational studies.
 
Tips on brewing tea. Choose a ceramic teapot, a covered cup, or a glass. Preheat both with hot water. A teapot with a built-in strainer will prevent leaves from entering the spout. To allow flavor to develop fully, brew tea loose, not in a tea ball or infuser. Different teas have different weights. With lighter weight teas, be sure to add enough leaf. Start with good-tasting water. Try different spring waters, or filtered tap water. Do not use boiling water when brewing white or green tea. Over-heated water will cook the leaves and destroy their flavor.
 
Finally, take the time to enjoy your cup of tea. It is an opportunity to savor the present moment. Notice the color of the tea, the aroma, how it feels in your mouth, and the warmth as it travels down your throat when you swallow. Like wine, you can become a tea connoisseur. By tasting different teas you can begin to detect the subtle (and not so subtle) characteristics of each one. If you are interested in tasting different teas go to the Imperial Tea Court in San Francisco. You can buy high quality teas from a number of websites including Silk Road Teas and In Pursuit of Tea (credited for much of this information).