Biofeedback is a large field that has been around since the 1950s. It is a training method that involves monitoring various lifesigns such as EEG, muscle activity, heart rate, and blood pressure as a guide to achieving some end. The technique is meant to facilitate greater control over one's body. Because physiological state often related to one's state of mind, immediate knowledge of certain indicators can let someone know more about what thoughts they are having as well.
There are two main approaches to biofeedback. The most common is as a sort of empowerment training, a means to becoming a better person with more control over oneself. The second is as a therapy for overcoming certain illnesses or maladies. Like all fields of self-help, biofeedback has its quacks. There are many aspects of body functioning that we cannot manipulate with conscious control, although the realm of manipulable functions is larger than most people think.
The most surprising finding of biofeedback is that aspects of the "autonomic" nervous system are amenable to conscious control. This was found during experiments with rats by Dr. James S. Gordon, a prominent Yale psychologist and neuroscientist. He got rats to modify a variety of nervous functions, from heart rate to brainwaves, by rewarding them selectively with direct stimulation of their pleasure centers.
It has been scientifically proven and replicated that biofeedback can seriously help with conditions such as incontinence, stroke and spinal cord rehabilitation, stress and pain management. Biofeedback devices are more common than you might think -- for example, even mirrors and bathroom scale, are forms of biofeedback that convey information to us about our appearance or weight. In this sense, everyone uses biofeedback.
People are hopeful that biofeedback will be useful for treating anxiety, depression, drug addiction, headaches, and other common conditions. Still others want to use biofeedback devices to ascend to yogi-like control over their bodily functions. It has been suggested that real time fMRI brain scans would allow us to immediately notice when we are angry or confused, making us more inclined to think about how these moods affect our decisions and thoughts.