JOB – Identifies the beginning of a job. The JOB statement informs the operating system of the start of a job, gives the necessary accounting information, and supplies run parameters. Each job must begin with a single JOB statement
EXEC – Indicates what work is to be done. EXEC (execute) statement is used to identify the application program, cataloged procedures or in-stream procedure that the job is to execute and to tell the system how to process the job.
DD – (Data Definition) Identifies what resources are needed and where to find them. A DD statement must be included after the EXEC statement for each data set used in the step. The system ensures that the requested I/O devices can be allocated to the job before execution is allowed to begin.