The caffeine in tea is called "theine" and it was discovered in 1827. Later studies shown that the "theine" of tea is identical to the caffeine of coffee, and the term "theine" was dropped. Altho the caffeine and theine are technically identical, the experience is different due to some key factors:
Tea contains more caffeine than coffee, when measuring their dry forms, but this is false when comparing brewed coffee and tea. One normally uses 2 grams of tea per 250 ml cup, and 10 grams of coffee for 100 ml of water.
There are of course also decaffeinated teas.