Hello and welcome. You should ask these questions of your co-workers. It will be part of your intooduction to your new job to be taught these things by them or to be sent on a course.
There is no such thing as a "JCL program". JCL is not a programming language - it is a means by which humans let the machine know what programs to run, in what sequence and what resources are needed by those programs.
OPC (TWS) can sumit jobs for many conditions - it is time to start, a predecessor job has completed, a dataset has appeared from somewhere (maybe by FTP) etc. Also, jobs that it monitors need not be submitted by itself but may have come from a remote location or some other job that that it did kick off.
What is confusing about the terms "long term", "current" and "trial"? those mean exactly what the words mean normally. But, the current plan is a subset of the long term plan for the current period. And guess what a "trial plan" is!
The problem I have is that people can explain things quickly but I can only comprehend slowly.