All padlocks within a set can be opened with the same key.
Keyed to Differ (KD)
Each padlock has its own unique key, one which cannot be used in any other padlock in the set.
Master Keyed (MK)
Each individual padlock can only be opened by a key specifically designed for that padlock, however there is also one key that can unlock all the padlocks in a set (this is a Master Key).