JOB vs EXEC vs DD - what each card is for

Absolute beginner JCL question. I see //JOB, //STEP1 EXEC, //DD everywhere. What is the job of each line?

JOB = names the job, accounting, class, etc. One per job.
EXEC = runs a program or PROC. You can have multiple steps.
DD = data definition. Connects a ddname to a dataset, sysout, or * for inline data.

Think: JOB is the envelope, EXEC is "run this", DD is "here are the files."