In a typical residential home, these are the different types of door locks:
For exterior doors, there are keyed door locks. These locks have a cylinder on the outside that, when locked, requires a key to be inserted to release the latch. The inside portion of a keyed entrance knob or lever will vary according to the manufacturer. For example, one manufacturer may offer a push button lock on the inside that releases when the knob or lever is turned, while another might offer a turn button on the knob or lever itself that must be unlocked manually.
Single Cylinder: The single cylinder function is an option for deadbolts. Single cylinder deadbolts have a keyed cylinder on the outside and a thumb turn to lock and unlock the door on the inside. The thumb turn on the inside allows for an easy exit in case of an emergency. Unless glass is present near the lockset, the single cylinder entrance lockset is preferable.
Double Cylinder: The double cylinder function is also an option for deadbolts. Double cylinder deadbolts have a keyed cylinder on both sides. They provide additional security but are usually not recommended because they make it more difficult to exit the home in the case of an emergency. They are, however, a good option for doors with glass in them or that are near a glass panel and are often used on garage doors and patio doors. Double cylinder deadbolts are sometimes restricted by local building codes.
Handlesets (or front door sets) are most commonly used on exterior entrance doors and do not lock on the handle part but are available with single cylinder or double cylinder deadbolts, or as a dummy set.
Single Door Entry: Usually if there is just one door, a handleset with a single cylinder deadbolt is sufficient. The deadbolt will lock with a key on the outside, and lock or unlock with a thumb turn on the inside.
Double Door Entry: For a double front door where one side is generally inactive while the other is always active, a combination of two handlesets is required.The active side will use a handleset with a single cylinder deadbolt. The active side will close and latch to the inactive side and the deadbolt will latch into the inactive door edge. The inactive door can be fitted with a dummy handleset.
Glass Door Entry: For a full, or even half glass front door, many prefer to install a double cylinder deadbolt. A double cylinder deadbolt is keyed on both sides, so rather than a thumb turn on the inside, it has a keyhole like the exterior does. The reason for this is that if a burglar were to break the glass, he still would not be able to unlock the door. However, this is not always recommended, as exiting the home quickly in the event of a fire means first locating the key to unlock the door. This option should only be used when there is another escape route easily accessible.
Passage/Hall/Closet Knobs and Levers: Knobs and levers for passage doors are non-locking - they do not have keyed cylinders or locking buttons. They provide unimpeded access to any room. Passage, hall, and closet knobs and levers are used on closets, laundry rooms, or other rooms that do not require a privacy lock.
Privacy/Bed/Bath Knobs and Levers: Privacy hardware is generally used on bedrooms and bathrooms, or anywhere that privacy is wanted but an actual keyed lock is not necessary. They provide secure locking from the interior. Locking features for privacy, bed and bathroom door knobs will vary according to manufacturer. For example, they might have a pin hole on the outside, and a push button lock on the inside that releases when the knob is turned. Other brands will have a turn button on the knob itself that has to be unlocked manually, and a pin hole on the end of the knob or on the rosette. In the case of an emergency, they can be unlocked using a generic key provided by the manufacturer that is inserted into the pin hole on the rosette or knob.
Dummy Knobs and Levers: Dummy knobs and levers are one-sided, with no working parts. They are usually surface mounted or mounted from behind. They can be sold in pairs to be used on sets of double doors. Dummy knobs are usually used on shallow closet doors, which may have a ball catch or magnetic catch to keep the door closed when not in use. Dummy door knobs and levers are also often used on the inactive side of a French door.