Locks that are keyed alike can all be opened with the same key. Keying alike is great if you need tobe able to open several doors (front door, back door and security door for example) with a single key.
A master key can open two or more locks that each have different keys. People who have a master key can open multiple doors that are keyed different with their master key. A master key system uses security key blanks which are specifically designed keys that can only be obtained / copied only from a locksmith or a business which designed the system, and then only with authorization from the lock's owner. Not all manufacturers offer master key service, but UAccess does for their Eternity (iButton) locks.