Answer:
The door entry system consists of an intercom (with or without a camera) - a switch and an electromagnet.
The electromagnet holds the door closed so long as current is flowing. When someone requests access (usually by use of a call button) they speak to the person inside the building via the intercom. When the person inside is happy to let the person in, they press a switch which disrupts the current to the electromagnet. This releases the door so the visitor can walk through. Doors usually have a powerful spring to close the door behind you when you walk through.