Keyed alike means the locks would operate using the same key, keyed different means the locks would operate using different keys.
Locks are classified as keyed alike when they have the same lock number and are by the same manufacturer. For example, if you have a 101E and a 102E lock they are keyed different. If you'd like to key them both to 101E, purchase another 101E lock core and a Control Key to remove the 102E.
Unfortunately if your locks are not by the same manufacturer or are not within the same lock series they may not be able to be keyed alike. If you are unsure if your locks can be keyed alike or have more questions just Contact Customer Service.