Hi.
Quant Psych is mostly composed of psychometrics, which is basically an area that creates mathematical techniques for other psychologists. Academics in this area generally work closely with people in educational psychology and test making, though there are other areas. A smaller area of quant psych is mathematical psychology. In addition to using statistics, mathematical psychology uses techniques from areas in math other than statistics to model human behavior. It is similar to applied math.
The difference is applied math is more general, and therefore more theoretical. Getting a PhD in quant psych (unlike in applied math) may involve working with real psychological data. A student in quant psych is trained in applied math specifically as it applies to psychology, while an applied math student is trained in applied math in general.
The best resource for finding out about these things seems to be applying to and visiting quant psych programs. The area is so small that they have trouble advertising themselves.