Draugrs are undead Nordic warriors found almost exclusively in dungeons. There are several different variants of draugrs. They are listed here from easiest to hardest.
Draugr: A basic zombie wielding a one-handed weapon or bow.
Restless Draugr: A more difficult version wielding a one-handed weapon and the ability to cast the frostbite spell.
Draugr Wight: Higher health than the Restless Draugr, carries a one-handed weapon and a shield. Can cast low-level destruction spells (usually frost spells, which have the annoying effect of slowing movement speed.
Draugr Scourge: Draugr that will usually carry either an ancient nordic bow and arrows, ancient nord melee weapons, . These items may be stronger versions of the weapons, i.e. honed, flawless, etc. First Draugr to know Unrelenting Force shout. If magic-wielder, specialized again with frost attacks. Known to occasionally cast conjuration spells.
Draugr Overlord: Simply a stronger version of the Draugr Scourge. Possibly with Frost Cloak or Disarm shout in addition to Unrelenting Force
Draugr Deathlord: Highest leveled Draugr found normally in dungeons. Will carry ebony weapons including ebony bows and arrows.
Dragon Priest: Magic-wielding Draugr that specialize in one field of destruction magic. Infrequently found in dungeons. Named ones with unique enchanted masks found near Word of Power walls in a Dragon Lair. e.g. Krosis at Shearpoint, North-east of Whiterun City