What is dbms job?

If I understand your question you are asking what a JOB is in say Sql Server 2000? A job, or the job component in SQL server is basically an engine that allows you to schedule objects to fire. As an example lets say you have a stored procedure that is setup to retrieve data from a table or set of tables, analyze it and split this data off into new tables, perhaps with additional information added, and maybe even delete some of the records that fit within certain rules. That said now you don't want to have to go and execute (run) this stored procedure on your own, manually; do you? Now you can setup a job that you will assign a schedule to (ie: Run every few minutes, every hour, once a day, whatever you want). And in this JOB or schedule you will attach this stored procedure. The SQL engine looks at the scheduled jobs, compares the last run time with when it is set to run again and then fires off whatever is associated with the job; in this example our stored procedure.