If you go to the Exercises section and watch the video called "Presses," you'll see exactly what a push jerk is. Coach details the 3 forms of presses: shoulder press, push press, and push jerk, in one of the CFJs from a while back (Jan 2003). Basically, the push jerk has you doing the same one-quarter to one-half squat at the beginning of the movement (just like for the push press), then driving the bar upwards. But instead of pushing and catching the bar overhead with straight legs (a la the push press), you catch it with bent legs.
The push jerk (and push press) is a great developmental exercise for the clean and jerk.