The jerk is just like a push press except when your legs are fully extended at the top of the jump, you bend them again and push your body under the bar. Heavier loads can be achieved with a push jerk because instead of pushing the bar up, you are pushing your body down. When you finish pressing the bar up and your arms are locked out, you must stand up straight before the lift is considered complete.
There are two types of jerk: the push jerk and the split jerk. In the push jerk, you land with your feet together, almost exactly where they started. In the split jerk, one foot moves forward and one moves back. The split jerk is more technical but generally allows larger loads because it is easier to get your body lower when your feet are split.
The obvious substitutions for a jerk are the shoulder press and push press. But the ability of a push jerk to achieve heavier weights means that at those weights the jerk will have a greater core strength demand than amost any other lift—because you simply won’t be able to do any other lifts with that weight.
As with any other barbell movement, the jerk has infinite scalability because you can always reduce the weight. You can drop all the way down to a PVC pipe if necessary. If you are looking for a substitution because you don’t have good enough technique, you should pull out the broom stick or PVC pipe and start learning. The jerk is the most efficient way to lift weight from the shoulders to overhead. It is an indispensable tool that everyone can and should learn.