Keyed to Differ: Individually keyed locks only opened by their own key.
Keyed Alike: A number of locks operated by the same key. For example you could have the same key to open both your front and back doors. In a shop you could have just the one key to open the security shutters & padlocks as well as the front and back doors.
Master Keyed: This is the clever stuff. Imagine a block of 10 flats with 4 offices, a toilet, staff room and various cupboards and boiler rooms. Each tenant has their own individual flat door key which will not open any other lock in the suite. However the owner or manager can have a master key which operates all of the locks in the suite. This means the owner/manager only need to carry 1 key in order to access the whole site without having to sort through 20 or 30 different keys every time they want to open a door. Other members of staff would have a key which only opens only their own office door. If you wish, you can mix keyed alike and master together so that the office staff have 1 key which opens all 4 of the offices but nowhere else.
Keyed to common: In the above master suite we can also have a common cylinder. This common lock could be fitted to the main entrance, toilet or staff room doors. Every key in the system will open the common door allowing the door to be kept locked at all times while still allowing all of the staff and tenants to be able to gain entry with their key.
Sub Master: It is also possible to have a sub master where every door has it's own lock and key but all of the maintenance cupboards, electric cupboards, boiler rooms etc are opened by a sub Master key. This sub master key could be held by the janitor allowing them access to those areas when needed but will not open any other doors. The owner/mangers Master key will still open all of these doors.
There are many other ways of mixing and matching master key suites, so its always best to ask a master locksmith to design the suite to your particular requirements.