Light Monsters are the historic opposites of Dark Monsters. Blue-Eyes White Dragon, the Components of XYZ-Dragon Cannon, and monsters such as D.D. Warrior Lady, and Watapon, are all Light monsters. Light monsters are represented often by Fairy, Spellcaster, Thunder, and at times Warrior-Types, but in general, they encompass a wide array of monsters.
Lights served as Tribute fodder for the two Envoys (Black Luster Soldier - Envoy of the Beginning and Chaos Emperor Dragon - Envoy of the End). They used to serve the same capacity in Chaos Decks, for Chaos Sorcerer, until its September 2006 ban.
Many light types focus on positively reinforcing the player instead of directly damaging the opponent, such as by increasing their Life Points or allowing the player to draw extra cards from their Deck.