Two or three of these basic questions, we will deal with. But after that, you'll need to find actual reference materials to answer such questions until you've learned enough to ask processing questions (I'm trying to do X, here's my code to do it, but I'm getting error Y so what's wrong with my code) instead of learning questions.
A transaction is a one-to-four character identifier that CICS uses to start a program. The transaction is tied to a program when defined to CICS and when you enter the transaction name into CICS, the associated program gets executed.
CICS associates a task number to each executing program, whether started by a transaction or one of the other ways a program can start in CICS. So a task may be -- but does not have to be -- an invocation of a transaction, while a transaction is just a code with a program associated with it.