Found this explanation. Also found one that says Green Tea has less caffeine.

There is much controversy surrounding caffeine and its effects. Taken in moderation, caffeine increases alertness, reviving the spirits of the tea drinker. However, caffeine has also been claimed to be a substance of abuse, with many people addicted to an early morning jolt, particularly from coffee. It is not surprising that heavy users of caffeine are also associated with other unhealthy practices, such as excessive smoking and poor diets.
Tea contains caffeine. Per pound of tea leaf, tea contains more caffeine than coffee per pound of bean, but because more ground coffee is typically used to brew a cup of coffee than tea is used for a cuppa, the average cup of tea contains less caffeine than the average cup of coffee. Caffeine does have a slight bitter taste distinct from the mouth-drying, puckery taste of tea, known as astringency, but one cannot generally tell the amount of caffeine based on the taste of the brew.

Caffeine contents in tea depend on the method or style of brewing, the type of tea, and the temperature of the water. Contrary to popular opinion, green tea does not necessarily contain less caffeine than black tea, although because green tea leaves are generally larger-leaf teas and from var. Sinensis Chinese shrubs, they will sometimes produce a beverage which has lower amounts of caffeine than black teas, typically produced from var. Assamica Indian teas.

Drinking tea should be an enjoyable part of a healthy, happy lifestyle. Low to moderate tea consumption, combined with avoidance of caffeine from other sources (such as many cola-type drinks and sodas, coffee, some non-prescription painkillers and stimulants, Maté or cocoa/chocolate) should limit caffeine intake. If this does not prove satisfactory, decaffeinated teas can be obtained, which would further reduce (but not necessarily eliminate) caffeine consumption.

More information on the safety of caffeine can be obtained from the International Food Information Council or from your physician. This advice is not intended as medical advice, and you should always consult your physician if you believe you are experiencing problems with caffeine or tea consumption.

Thank you very much! That was what I was looking for, and it was very interesting as well!

green tea has less than black tea

In general, white teas are the lowest, and contain only a fraction of the caffeine of coffee. Green teas usually are low in caffeine, and black teas usually contain much higher amounts, but still average less than most coffee. A large percentage of the caffeine may be washed away from the tea leaves in the initial infusion of two hundred degree water.