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.

There is an abstract for a paper describing the differences available on the the web.