Pd is a free, open-source real-time graphical programming environment for audio and graphical processing that is very similar to Max/MSP. In fact, it is developed and is maintained by the original developer of Max, Miller Puckette, at UCSD. There are also many developers that contribute to Pd from its user-community.
The only visual indication of whether a patch is in "edit" or "run" mode is via the cursor (toggle with CTRL-e).
Pd does not conform to the right-to-left ordering of output messages. Instead, outputs will happen in the order they were created. It is expected that users will make use of the trigger object to disambiguate message ordering.
Pd has less graphical user interface objects than Max.
Various objects in Max are not in Pd and vice-versa.
In Pd, inlets and outlets in subpatches are named objects, not GUI elements (as in Max).
Pd uses floating-point representations for all computations.
There is no "reference manual" describing all the available Pd objects. However, you can open a "help" window by right-clicking on a specific object or use the "Help" menu to view program documentation.