A higher-studies NOC states that your employer has no objection to you enrolling in a course alongside your job. Universities want it to confirm no conflict with your working hours, so name the programme, the institution and the schedule.
A short letter from your employer stating they have no objection to something you are doing — travel, study, a second job, a loan application. It is not a legal permission; it is documented awareness and consent.
HR, on company letterhead, with an authorised signature and usually a company stamp. Your line manager's email approval is not normally accepted by embassies or banks.
Typically three to ten working days. Ask early, especially for visa appointments, and give HR the exact addressee and wording the receiving body requires.