Many times my customers ask me does XXX tea contain caffeine and if yes, how much? Tea does contain caffeine, which is reasonably low compared to other products. For example, I cup of coffee has the same amount of caffeine as 4 tea bags of green tea (4 or more cups.)
Here is the details I found on the net: (Everything in miligrams of caffeine)

Coffee (Average per serving 80 in the range of 40 to 190)
Cola (12 oz can) (Average per serving 45 in the range of 30 to 60)
Black Tea (one tea bag) (Average per serving 40 in the range of 25 to 110)
Oolong Tea (one tea bag) (Average per serving 30 in the range of 12 to 55)
Green Tea (one tea bag) (Average per serving 20 in the range of 8 to 30)
White Tea (one tea bag) (Average per serving 15 in the range of 6 to 25)
Decaf Tea (one tea bag) (Average per serving 2 in the range of 1 to 4)
Herbal Tea(one tea bag) (Average per serving 0 in the range of 0 to 0)

So to compare, it takes 3 oolong tea bags to be approximatelly equal in content of caffeine as 2 cans of cola. 3 tea bags would be enough for me to make a half gallon of cold tea :).

Also, does caffeine in Tea and Coffee is dangerous for us? Well, there was some research sponsored by U.S. Food and Drugs Administration stating:
During the past decade, extensive research on caffeine in relation to cardiovascular disease, fibrocystic breast disease, reproductive function, behavior in children, birth defects, and cancer has identified no significant health hazard from normal caffeine consumption.

Here is more information about caffeine:

Black tea
Contains caffeine in nature, can be decaffeinated (removing most of the caffeine)
Examples: English Breakfast, Chai Spice, Earl Grey

Oolong tea
Contains caffeine in nature, generally less than black tea, can be decaffeinated (removing most of the caffeine)
Examples: China Oolong, Formosa Oolong Bay Jong, Ti Kuan Yin

Green tea
Contains caffeine in nature, generally less than oolong tea, can be decaffeinated (removing most of the caffeine)
Examples: Premium Green, Kangra Green Gunpowder, Darjeeling Green

White tea
Contains caffeine in nature, generally less than green tea, can be decaffeinated (removing most of the caffeine)
Examples: Mutan White, Flowery Pekoe White, White Pearls

Decaffeinated tea
Results of removing most of the caffeine from black, oolong, green, or white tea
Examples: Decaf Earl Grey, Decaf Chai Spice, Decaf Sencha Green

Herbal "tea"
Naturally caffeine free
Examples: Chamomile, Peppermint, Lemon Blossom

 If you are interested in buying good quality of tea, please visit my store at http://www.stores.ebay.com/expotea
This article is based from my blog about tea; however, due to eBay policy I cannot include link here. If you are interested to read more, feel free to email me.