TPACK stands for Technological, Pedagogical, and Content Knowledge.  Technological Knowledge is using being able to use technology.  Pedagogical Knowledge is how to teach, and Content Knowledge is the knowledge about a particular subject, like science.  It is ideal when teachers teach with all types of knowledge.  It is best when teachers know there subject well, know how to teach it well, and use technology in their teaching.  When students can use the technology it is even better.  An Engineer might have content knowledge, but perhaps not pedagogical knowledge because he does not teach it.  If he were a Engineer Professor, maybe he would have both pedagogical and content knowledge.  When he effectively uses Technology, then he has all components of TPACK.