A post office is a facility (in most countries, a government one) where the public can purchase postage stamps for mailing correspondence or merchandise, and also drop off or pick up packages or other special-delivery items.
Post offices also rent post-office boxes to people and businesses who prefer not to have mail delivered to their home or office.
In many countries, post offices include other functions, such as a place to pick up various government forms, to apply for passports, to send money to others, etc. In some countries, the post office functions as a financial bank and/or a central place to use public telephones.
The back rooms of a post office are where mail is processed for delivery. Mail may also be processed in other post offices that are not open to the general public.