A travel NOC confirms your employer knows about and permits your trip, and that you are expected back at work. Embassies use it as evidence you will return, so include your approved leave dates and your job title.
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.