Door Entry Systems are a method of security protecting a building from unwanted or unauthorized visitors.
There are used in a number of different situations like housing and places of work.
Most commonly you will see these being used in tower blocks.
A tower block is a set of flats in a tower; a door entry system will be installed at the entrance so only residents of the tower may enter the building.
The residents will be given a key fob, key card or a code, depending on what type of entry system is installed.
When the card or code has been input it will send a signal to the system, if correct the door will open and if incorrect the door will remain shut.
Another place these are very common are places of work.
They are often installed here at different intervals sectioning off different parts of the building.
Some employees may be given access privileges over other staff members depending on their position.
One way of effectively doing this is adding door entry systems on the doors with key cards that have different levels on them.
Lets say there are 5 different entry points, 5 key cards will be made from Levels 1 - 5 and handed out accordingly to the correct staff members, this ensures that only the correct people have access to enter that section of the building.
Of course like most electronic devices there are ways around them like people lending their cars to others etc, so 1 extra security method that can be used are key cards and codes used in conjunction with each other, this doubles security.