Which are the most common types of keys?
A standard key is the most common type. It is held in a vertical position and inserted into the cylinder. A set of pins lifts up and line up with the key teeth and grooves. If the inserted key is the right one, the length of the pins matches with the length necessary to allow the rotation of the cylinder which, in its turn, unlocks the door.
It differs from the standard keys because it is held in a horizontal position. Coding is not made through the key teeth and grooves but by small drills of different depth on the key section.
A double-bit key is very similar to a key for home use or to a car key, except for the presence of an upper set of teeth and a lower set of teeth. It is inserted into the lock and moves the dead bolt directly.
A Master key opens several cylinders (sets of locks). These locks also have single keys which are specific to each one and cannot open any of the others in the set. Locks which have master keys have a second set of the mechanism used to open them which is identical to all of the others in the set of locks. Big organizations may have "grandmaster keys" opening locks which can be opened by different master keys.