Balor is often used as a generic name for a demonic enemy, usually bosses, but in Castlevania and the Megami Tensei series he is depicted as a being closer to how the original Celtic myth describes him.
In Castlevania, Balor (or Balore) is a huge humanoid entity that takes up a large portion of the room and attacks largely with his fists. When pushed he'll also attack with his legendary eye - said to kill a normal man dead should it make contact, and usually depicted in Castlevania games as a roving laser beam. The eye is also his weak point. He's been a boss in every Castlevania game he's appeared in.
Shin Megami Tensei
Balor is of the extremely strong Tyrant clan of Strange Journey and appeared in the GBA DemiKid games as "Evil Eye". He is depicted as seen on the right; a loose interpretation of the Fomorian.
Dungeons & Dragons
The balor, or Type VI Demon, are the most powerful non-unique or named demon-type (that of the Chaotic Evil Tanar'ri from the Abyssal Plane) enemies in the D&D universe. They attack chiefly with magic, their mastery of flames and their sword and whip weapons.
A named balor, Belial, appears in Neverwinter Nights should the player summon him with the correct procedure in Chapter 2. Belial will inform the player of another NPC's evildoing before vanishing, but only if the player remembered to cast Protection From Evil on their party. Attempting to fight a balor at that stage of the game will not go well for the player.
All D&D balor are modelled after and inspired by J.R.R. Tolkien's Balrog - another powerful demonic entity. One, Errtu, has a legendary antagonism with Drizzt Do'Urden the Drow ranger.