Job statement is to execute a job. It identifies the
Beginning of the job. Without this job statement, the
operating system will not execute the job.

The various keyword parameters are:

CLASS - it identifies the nature of the job that is to be
submitted
PRTY - assigns priority to the job
MSGCLASS - determines the output device to which system
messages and jcl messages are written
MSGLEVEL - specify the jcl and the allocation messages 
which will be recorded on the output device specified in 
the msgclass
TYPRUN - this tells whether the job is to be held until it
is released or its execution is to be suppressed altogether

Example:

//JOB1 JOB
(A123),'KRITHIGA',CLASS=E,PRTY=1,MSGCLASS=A,MSGLEVEL=(1,1),TYPRUN=HOLD