5-HTP is a more recently popularized version of Tryptophan which was banned in the early 1990's I believe. This was due to a problem in the manufacturing process which was causing toxicity (read: people were dying) in the end product.
You may wonder why we use 5-HTP to increase seratonin in the brain. Why not just take serotonin? Well, the answer is that serotonin can not cross the blood / brain barrier.... its molecules are too large. Taking serotonin in a pill would only cause it to float around in the blood stream, never to end up in the brain where it belongs. 5-HTP is a precursor to serotonin, passes the blood / brain barrier and is converted in the brain.
In regards to its effect in the brain, 5-HTP increases the available supply of serotonin. This is helpful in case of serotonin depletion due to drug abuse (MDMA) or high levels of stress. However it is significantly different from drugs such as Prozac or Zoloft which alter the uptake rate and release rate of seratonin from neuroreceptors.
Briefly, once serotonin is released, it will bond to a neuroreceptor in the brain, creating a sensation of pleasure. In people who are depressed, there is one of three things gone wrong: 1) Not enough serotonin. 2) Serotonin is not effeciently attached to the receptor site. 3) The serotonin is released from the receptor too soon. Prescription drugs such as those previously mentioned work on the latter two problems, while 5-HTP addresses the first.
My personal experience with 5-HTP stems from about 2 months of research followed by a daily regimen of 100MG 2-3X per day for 3 months. I can say that regardless of who you are, you will feel a difference while taking this supplement. I do caution jumping in to this hig of a dose right off the bat, as headaches are not uncommon for 'overdose'. If you want to try it, start off at 50mg 2X per day and increase gradually to desired effect. 5-HTP is not a cure for a lousy life.