Mormon tea is various species of plants of the Ephedra genus. I know of three species in the Western US, but they all look very similar and are all known as Mormon Tea. The species in Asia, from which the herb Ephedra that you might have found in a store several years ago before it was banned, has a stronger amphetamine in it than the North American species do. The Asian version can cause a lot of the same problems as amphetamine addiction, but the North Amercan "Mormon Tea" is much milder. It does have an uplifting affect similar to coffee or tea, though, just not as strong. It has a definite different chemical composition.
I've read that Mormon Tea was used to fight syphilis, and was served to guests in the houses of ill repute. Another common name you will find it sold under is "Brigham Tea", I guess because some one doesn't like to use the word "Mormon" with it.