Quote from the PD manual: "Pd is a real-time graphical programming environment for audio and graphical processing. It resembles the Max/MSP system but is much simpler and more portable".
I think this is only interesting if you're into experimenting with a wide range of possibilities and no fear for sitting long nights in front of your computer, pushing your mouses "used kilometers" to the heights... but the results can be VERY rewarding (my personal experience)