Pd is ultimately a redesign of an earlier system that is still in commercial development: Max/MSP. In fact, the real time audio synthesis portions of MAX/MSP incubated in the Pd project. jMax then is a version of Max released as open source by IRCAM a while ago. It seems to be unmaintained, at least not a lot of people still use jMax. Pd and Max projects were masterminded by Miller Puckette and others at IRCAM in France.

If you know Max and want to get started with Pd, there is a Tutorial Pd for Max Users