The difference between TPACK and TPK is that TPACK is technological pedagogical and content specific knowledge whereas TPK is just technological and pedagogical knowlege. In my discipline of theatre TPACK seems that it would be more helpful to the students in learning more about my content area and TPK would just be something fun and interesting for them to do. It might be teaching them a cool new technological tool but it doesn't help further their knowledge on the subject area.
In theatre, an example of TPACK would be teaching the students how to use a sound editing program to create the sound clips for a play that they are designing. This would be TPACK because it includes pedagogy in teaching how to do it, technology in using a software program to edit the sound clips, and content of utilizing sound clips in a theatrical production. An example of TPK would be having the students create a slideshow with their pictures and videos from the class at the end of the year. Again this would include teaching and using technology, and pedagogy in the way that it was introduced to the students, but it lacks in having something content related.