Wait - is DragonForce a serious band or are they a parody?
Yes.
Doom/Stoner/Sludge are all technically different (but the differences are splitting hairs mostly) but are usually used as synonyms to describe a type of metal that while heavy is usually super slow and loves to drone. Lots of songs are longer (an extreme example is the song “Dopesmoker” by Sleep Which at over 60 minutes in length stretches over 2 sides of an LP) ultimately in its essence it has guitars that are played low and slow (drop tuning and slow melodies) which gives off a very “Heavy” and repetitive sound.
For my money, the best Doom album of all time is “Dopethrone” by Electric Wizard
Doom is a direct descendent of Black Sabbath. Slow. Fuzzy. Heavy. Usually songs about death/gloom/hopelessness. There are even subgenres OF doom.
Traditional Doom - Black Sabbath and the like.
Epic Doom has soaring melodic (sometimes super high register) traditional heavy metal vocals. See: Candlemass
Gothic Doom is a blend of goth rock and doom. See: Type O Negative
Stoner doom (more focus on grooves, fuzz and psychedelic soundscapes and also frequently songs about weed). See: Sleep, Kyuss
Funeral doom is exxxtreeeeemely slow. Crushingly heavy and plodding often with harsh vocals/growls. See: Bell Witch
Death-doom is a hybrid of death metal and doom. See: Early Katatonia, Hooded Menace
Sludge is punk infused doom. Faster pace and more aggression, less doom and gloom. See: Acid Bath, Crowbar
Drone doom is a blend of drone and doom. See: Sunn 0))
Plenty of mixing and matching commences as well. Progressive elements have been used in all of the above (ie progressive sludge in Mastodon).