Djinn of Rituals, known as "Ritual Genie" (儀式魔人Gishiki Majin) in the Japanese version, is a series of DARKFiend monsters released in the Booster PacksStardust Overdrive and Absolute Powerforce, and gained another member on Duelist Alliance. They each share the effect text "When you Ritual Summon a Ritual Monster, you can banish this card from your Graveyard as one of the monsters required for the Ritual Summon.", followed by their individual effects, which are given to the Ritual Monster they were used to Summon. The purpose of these Monsters is to make Ritual Summoning more viable.
Visually, all Djinn of Rituals monsters resemble strange, obese humanoids wielding weapons or magical tools. Their bodies are usually one single predominant color. Every Djinn has a skull somewhere on their weapon and each Djinn has a different Level and effect, having a dynamic engine to summon and boost Ritual Monsters:
The Ritual Monster can't be targeted by your opponent's card effects
Tips
Ritual Spell Cards that Ritual Summon a specified monster (for example, "Black Illusion Ritual") don't require you to use the exact Levels for the Ritual Summon. However, you can not banish extra "Djinn" Monster Cards beyond those which would be sufficient for any given Ritual Summon.
The last 2 can make setting up a Ritual Summon insanely quick. By summoning "Armageddon Knight" and then having him drop a Djinn, you're instantly set up for just about any Ritual Monster you like.
The Djinn also have decent ATK/DEF scores in their own right, so don't be afraid to simply Summon them. Plus, even if they are destroyed, you can still use them for Ritual Summons.