Necron Lords used to be actual lords, back when the Necrons were still fleshy and called Necrontyr. Presently, each Lord rules over a single Tomb World and all of its legions and phalanxes, assisted by a court of lesser nobles, and is himself a part of an Overlord's Royal Court (along with the Lords of neighboring Tomb Worlds and a few Crypteks as well). Because they were nobles, they were given higher-quality bodies and their minds were left quite intact by the biotransference, and so they are free-willed individuals, with names and personalities. That said, the millennia in stasis have done strange things to their minds, to say the least.
The Necron Lord used to be the only customizable HQ choice available to the Necrons (the non-customizable HQ choices being the Deceiver and the Nightbringer), but was otherwise devoid of character (to the point that, in Dawn of War: Dark Crusade, the "voice" of the Necrons was a Pariah). With the infusion of character brought by the Fifth Edition Codex, Lords are now individuals in their own right (with names and everything), but with the new layers of rulers above them, they are not available on their own anymore; instead, you get up to five of them as a "Royal Court" for each Overlord in your army, and can parcel them out to lead a squad of Warriors, Immortals, Deathmarks, or Lychguard, rather like a Sergeant or Wolf Guard.
The 7e updated codex brought them back as a HQ choice in their own right, though.
Some Lords go crazy after the sixty-million-year-long sleep, and get a bad case of sour grapes for being stuck in a machine for so long. They decide that they never really liked being alive anyway, and that everyone should join them in death as well. To that end, they modify their bodies to have a hoverboard base and an enhanced internal structure, much like the regular Destroyers.
Destroyer Lords are more machine-like than standard Necron Lords and are said to be the most maniacal of their kind, lacking any ability to empathize with other creatures due to their cold, calculating nature. Even mighty Necron Overlords find Destroyer Lords somewhat disconcerting, believing they have too willingly embraced the machine and may turn on their own kind when the galaxy is rid of organic life. As a result, many are outcasts and pariahs in Necron society.
In combat, it is said that the physical might of a Necron Destroyer Lord equals the mightiest Overlords. Whilst a Destroyer Lord dwarfs even that in terms of madness and all-out savagery, which would explain why other Necron leaders shun them. Most favor warscythes and voidblades.
Destroyer Lords serve no Overlord or Phaeron (and are a stand-alone HQ choice), because the less-crazy nobles are worried that the Destroyer Lords will turn on their fellow Necrons once all organic life is wiped out, and because they're afraid that Destroyer-ness may be contagious.
Ninth edition adds the Skorpekh Lord, who is simply a Destroyer Lord of the new Skorpekh variant.
When the Necrons were first introduced in Second Edition, the Necron Lord miniature was a scowling skele-bot with a goofy headdress (not that the other Necron miniatures were less silly). The miniature was later updated to be sleeker and have more options, though the old design was kept around as the "Classic Necron Lord" and updated to Finecast when the new Codex came out. It is now no longer available.
- Necron Lord of All Kaurava, an ancient (even by Necron standards) lord who want those fucking kids to shut up.
- Assholetep the Insufferable, a tantrum-prone Classic Necron Lord.