A Grand Master is the leader of the lodges within his Masonic jurisdiction. He presides over a Grand Lodge, and has certain rights in the constituent lodges that form his jurisdiction.
Just as the Worshipful Master of a lodge annually appoints lodge officers to assist him, so the Grand Master of each Grand Lodge annually appoints grand lodge officers to assist him in his work. Grand Lodges often elect or appoint Deputy Grand Masters who can act on behalf of the Grand Master when he is unable to do so. In English Freemasonry, where a member of the Royal Family is often the Grand Master, he may also appoint a Pro Grand Master to deputise for him when he is involved in affairs of State. The Pro Grand Master has no function when the Grand Master is present, and is distinct from the Deputy Grand Master.
There are two distinct traditions in connection with the office of Grand Master. Generally speaking the European practice is for the same Grand Master to be re-elected for several consecutive years, maybe even several decades, whilst in other countries a Grand Master serves a set term of only one to three years, and then retires.