Picking a lock can be a minefield.
You first need to decide what level of security is required. Then how you want your lock to work. Described below are some different types of locks.

SASHLOCK
This is a lock that is used with door handles it has a deadbolt, which is drawn and withdrawn, by a key and a latch bolt, which is operated by your lever handle or knob.
The main sizes are 63mm and 76mm; it also comes 3 lever (mainly locking internal doors and low security areas) and 5lever insurance standard (for locking front and rear doors to buildings)

DEADLOCK
This comes in all of the variations that the sash lock does but it does not have a latch bolt and therefore needs no furniture to operate it. (It is commonly used as a secondary lock in areas that you require extra security)
SASHLOCK CASE
As per the sash lock it does everything the same and comes in the same sizes. Where the real difference comes is the versatility that you get. Instead of a simple key there is a hole in the lock case in which to insert a cylinder, which can perform various functions (these will be described later).
DEADLOCK CASE

AS above the deadlock case does everything the sash lock case does but does not have the latch bolt and can be used without furniture CYLINDERS

If you have a lock case then you need a cylinder. These come in two main shapes and various sizes as described below.

DOUBLE CYLINDER
A key from both sides to lock/unlock operates this cylinder. A bonus is that if anyone loses a key rather than replace the whole lock only the cylinder has to be replaced.
CYLINDER AND TURN
This cylinder has a key operation from one side and a thumb turn operation from the other (this enables the user to lock or unlock the door without a key)
SINGLE OR HALF CYLINDER
This cylinder has only one end and is usually half the size of a full cylinder (very useful if you have a door that you only want locked or unlocked from one side).