A post office is a public facility and a retailer that provides mail services, such as accepting letters and parcels, providing post office boxes, and selling postage stamps, packaging, and stationery. Post offices may offer additional services, which vary by country. These include providing and accepting government forms (such as passport applications), and processing government services and fees (such as road tax, postal savings, or bank fees). The chief administrator of a post office is called a postmaster.
Before the advent of postal codes and the post office, postal systems would route items to a specific post office for receipt or delivery. During the 19th century in the United States, this often led to smaller communities being renamed after their post offices, particularly after the Post Office Department began to require that post office names not be duplicated within a state.
The term "post-office" has been in use since the 1650s, shortly after the legalisation of private mail services in England in 1635. In early modern England, post riders—mounted couriers—were placed, or "posted",every few hours along post roads at posting houses (also known as post houses) between major cities, or "post towns". These stables or inns permitted important correspondence to travel without delay. In early America, post offices were also known as stations. This term, as well as the term "post house", fell from use as horse and coach services were replaced by railways, aircraft, and automobiles.
Today, the term "post office" usually refers to government postal facilities providing customer service. "General Post Office" is sometimes used for the national headquarters of a postal service, even if the building does not provide customer service. A postal facility that is used exclusively for processing mail is instead known as a sorting office or delivery office, which may have a large central area known as a sorting or postal hall. Integrated facilities combining mail processing with railway stations or airports are known as mail exchanges.
In India, post offices are found in almost every village having panchayat (a "village council"), towns, cities, and throughout the geographical area of India. India's postal system changed its name to India Post after the advent of private courier companies in the 1990s. It is run by the Indian government's Department of Posts.  India Post accepts and delivers inland letters, postcards, parcels, postal stamps, and money orders (money transfers). Few post offices in India offer speed post (fast delivery) and payments or bank savings services. It is also uncommon for Indian post offices to sell insurance policies or accept payment for electricity, landline telephone, or gas bills. Until the 1990s, post offices would collect fees for radio licenses, recruitment for government jobs, and the operation of public call telephone (PCO) booths. Postmen would deliver letters, money orders, and parcels to places that are within the assigned area of a particular post office but there are no post offices in the location. Each Indian post office is assigned a unique six-digit code called the Postal Index Number, or PIN. Each post office is identified by its PIN.
Private courier and delivery services often have offices as well, although these are usually not called "post offices", except in the case of Germany, which has fully privatised its national postal system.
As abbreviation PO is used, together with GPO for General Post Office and LPO for Licensed Post Office.
There is evidence of corps of royal couriers disseminating the decrees of Egyptian pharaohs as early as 2400 BCE, and it is possible that the service greatly precedes that date. Similarly, there may be ancient organised systems of post houses providing mounted courier service, although sources vary as to precisely who initiated the practice.
The Roman historian Suetonius credited Augustus with regularising the Roman transportation and courier network, the Cursus Publicus. Local officials were obliged to provide couriers who would be responsible for their message's entire course. Locally maintained post houses (Latin: stationes) privately owned rest houses (Latin: mansiones) and were obliged or honored to care for couriers along their way. The Roman emperor Diocletian later established two parallel systems: one providing fresh horses or mules for urgent correspondence and the other providing sturdy oxen for bulk shipments. The Byzantine historian Procopius, though not unbiased, records the Cursus Publicus system remained largely intact until it was dismantled in the Byzantine empire by the emperor Justinian in the 6th century.
The Princely House of Thurn and Taxis family initiated regular mail service from Brussels in the 16th century, directing the Imperial Post of the Holy Roman Empire. The British Postal Museum claims that the oldest functioning post office in the world is on High Street in Sanquhar, Scotland. The post office has functioned continuously since 1712, during which horses and stagecoaches were used to carry mail.
In many jurisdictions, mailboxes and post office boxes have long been in widespread use for drop-off and pickup (respectively) of mail and small packages outside post offices or when offices are closed. Germany's national postage system Deutsche Post introduced the Pack-Station for package delivery, including both drop-off and pickup, in 2001. In the 2000s, the United States Postal Service began to install Automated Postal Centers (APCs) in many locations in both post offices, for when they are closed or busy, and retail locations. APCs can print postage and accept mail and small packages.