The term Lord is an honorific used to indicate that a man or his father has rank. It is used for a marquis, earl, count, viscount, baron or baronet or the son of duke or marquis. It came to be extended to non-peerage ranks, eg. Lord Mayor or Lord Chamberlain, and church ranks, eg. Bishop of Upper Smithbury, Lord Clancy.
For landed peers, it is used with the name of their estate, not their first name. Hence, you could refer to the Earl of Cumnor as Lord Cumnor.
A Lord in the SCA