"Lord" is a now extinct creature type, although the flavor of it still exists. Lords were created right as the game began and they are all defined as giving +1/+1 to all other creatures of their type. Lords are popularly used in tribal-based decks, as they often act as a Glorious Anthem with extra abilities.
Originally the lord cards, such as Lord of Atlantis and Goblin King, did not even have the creature type of what they helped, because the game did not yet support multi-type creatures. Eventually they were changed to have the creature type, but only help other creatures of that type. This kept them as essentially the same cards by themselves, but allowed them to count for all other tribal-based cards that such decks were likely to include. For example, Lord of the Undead would still help out all the other zombies, but one Lord of the Undead would be able to target another Lord of the Undead in your graveyard as well as count for Shepherd of Rot (create).
In 2007, Wizards of the Coast conducted the "Grand Creature Type Update" to change types of a great many creatures for many reasons, one of which being "if it has a non-descriptive type in its name, it should probably be that type." This meant that the non-Lord cards would become lords without any kind of tribal ability. This did not sit well with Wizards, so they opted to just remove the Lord type altogether. Ironically, the next set of the time, Lorwyn, had created new lord cards for every supported creature type.
- Note that only creatures which give +1/+1 to all others of its type are considered "lords." Creatures which give benefits to all or just use a lot of that type, while probably existing in the same decks, are not counted as lords. Also, slivers do not count as lords because, if they have any abilities, they always share them with other slivers, anyway.