For a piece of hardware few of us give much thought to at all, door handles are the product of mechanical and locksmith engineering and are more complicated than their brassy exteriors may have you believe. Door handles and knobs without keyed locking mechanisms have all the components listed below, save for the cylinder, which operates the lock.
Knob or Handle
You touch them every time you go in or out of your house. Although they can be lever-shaped or round and made out of everything from alloys to glass, the handle has two primary functions: It serves as a handle when you open a door, while it also slides the bolt from the door frame and releases the door from its closed position.
In the case of door handles that aren't rotating door knobs, a thumb piece is mounted above the handle as a means to slide the bolt out of the door frame. Spring loaded, thumb pieces automatically return to the closed position when pressure is released from them, extending the bolt to hold the door closed again.
Sometimes referred to as the spring latch, the piece that extends outward from the side of your door into the door frame is the bolt. Bolts are attached to the handle through simple levers and retract when the handle is turned. A properly tuned bolt will be spring-loaded to return to the closed position on its own, and bolts serving as the primary closing mechanism on your door (not deadbolt locks) are contoured to allow them to retract on their own when you close your door.
Sometimes referred to as rosette plates, these plates are the metal fittings that screw together on both sides of your door to hold the door handle in place and keep it secured from the outside. The plate that surrounds the bolt on the edge of a door is also an escutcheon plate.
If your door is fitted with a lock accessible with a key, as with most exterior doors, a cylinder operates the lock. Able to be swapped out of door handles without replacing the full hardware, cylinders have a set of pins of different lengths that turn and release the bolt when the key is inserted. American-style cylinders mount to the escutcheon, while European styles extend between escutcheons and are affixed to the door by a screw inside the door.