A gate, or gatehouse, is an area of an airport that provides a waiting area for passengers before boarding their flight. While the exact specifications vary from airport to airport and country to country, most gates consist of seating, a counter, an aircraft entry or exit doorway, and a jet bridge.

Small airports, where only a few flights a day leave, have enough at one gate, but most airports have multiple gates. The travelers are told at which gate they should report.

At the gate there is usually a waiting area where the travelers gather until they are allowed to enter. Sometimes several gates have a common waiting area. When boarding they have to show the boarding pass . Sometimes the boarding pass must already be shown before the passengers enter the waiting area.

The aircraft is preferably parked near the gate so that passengers can reach the aircraft on foot. Almost all international airports have flight barriers , creating a direct covered loop connection between the gate and the aircraft. Even then, the plane must of course be parked at the gate. If the aircraft is at some distance, a bus can be used to take passengers from the gate to the aircraft.

Domestic vs international
At most gates a single doorway opens to a jet bridge which leads to the aircraft door depending on aircraft model. If the gate is being used for departures,domestic arrivals or international arrivals coming from airports with pre clearance at that airport the door that goes to the waiting area will be opened and the hallways to customs will closed, preventing passengers from entering into the customs hall. For international arrivals that is coming from cities that does not have pre clearance, the door leading to the waiting area is closed and passengers are directed to a hallway where they can either go into the customs hall, if they end at that city, or clear customs and pre-board security, if they have a connecting flight.

Jet bridge vs airstair
Before the era of the jet bridge, or jetway, airline passengers embarked onto the aircraft from ground level via airstairs. If initially indoors, passengers would exit the waiting area through a door to the outside and then passengers would proceed to the airstairs leading to the aircraft door. This method is still used for boarding smaller planes or boarding at smaller airports.

Ownership
The equipment is either airport or airline property, in most cases airport infrastructure.

Procedure
After entering the respective terminal, the passengers at the check-in counter of their airline on presentation of the personal documents and, if necessary, their airline ticket on the baggage. You will then receive your boarding pass with seat and flight number and proof of checked baggage. The piers are equipped with or without security and passport controls depending on the intended use. Passengers can wait for their departure and on international flights in a waiting area at the gate duty free (Engl. Duty-free ) shopping. At the exit after the call for boarding (German entry into the aircraft) control of boarding passes. This will ensure that the passenger and baggage make the right flight.

The passengers then arrive at the aircraft in three ways:

If an aircraft is parked directly at the terminal building, it can be connected to the terminal with a movable arm ( passenger bridge or short finger ). Through this passage, the passengers arrive directly from the waiting area in their aircraft and vice versa after the arrival of the aircraft in the arrivals area or baggage claim . The finger is moved to the front door of the aircraft after being parked by an operator using a hydraulic control. Larger airports also have several fingers on a gate , which allow wide-bodied aircraft to simultaneously board passengers through multiple doors.
In the second variant, the exit consists of simple doors that open to the apron . Passengers arrive through these doors to an apron bus , which brings them to the aircraft. This will prevent passengers from walking down the apron to their aircraft, which may be parked a few hundred meters from the terminal building. In this case, the passengers board the aircraft via aircraft-owned or approached stairs (English gangway ).
Especially at small airports and low-cost airlines, it also happens that passengers from the pier on foot – usually via a closed way – go to the aircraft. As a result, the airline saves the charges for passenger boarding bridge and bus.

Source from Wikipedia