When the word Lord occurs in lowercase, the translator is indicating to us that the word adonai is found in the Hebrew Bible. Adonai means “sovereign one.” It is not the name of GOd. It is a title for God, indeed the supreme title given to God in the Old Testament. When LORD appears in all capital letters, it indicates that the word Jahweh is used in the Old Testament. Jahweh is the sacred name of God, the name by which God revealed Himself to Moses in the burning bush. This is the unspeakable name, the ineffable name, the holy name that is guarded from profanity in the life of Israel. Normally it occurs only with the use of its four consonants – yhwh.
– R.C. Sproul, The Holiness of God, Tyndale House Publishers, Carol Stream, Ill., Copyright 1985, Kindle Edition