Ritual Spell Cards known as Ritual Magic Cards in the OCG are used to Ritual Summon Ritual Monsters to the field, and require the following conditions to be met in order to be activated:
-The correct Ritual Spell Card in your hand.
-The corresponding Ritual Monster in your hand.
-Enough monsters in your hand, on your side of the field, or both, that satisfy the Ritual Monster's Level Requirements, to be used as Tributes. (The exceptions being "Advanced Ritual Art", "Forbidden Arts of the Gishki" and "Gishki Photomirror".)
-A free Monster Card Zone into which to Special Summon the Ritual Monster.
Activating a Ritual Spell Card can tend to be somewhat demanding, and for that reason Ritual Spell Cards are arguably the least used Type of Spell Card. The only Ritual Spell Cards that have even seen minor use tend to be 'universal' types such as "Contract with the Abyss", which can summon any DARK Ritual Monster. However, the only Ritual Spell Card that has seen major play is "Advanced Ritual Art", due to it being able to use monsters from the Main Deck, thus being much less demanding than traditional Ritual Spell Cards. However, as this card fueled the then-powerful Demise OTK Deck, "Advanced Ritual Art" has since been Semi-Limited.
Starting in Stardust Overdrive, Ritual Monsters appear to be making a return to the game, with a few new mechanics being added. Of note here, the Ritual Spells for "Divine Grace - Northwemko" and "Garlandolf, King of Destruction", "Ritual of Grace" and "Ritual of Destruction", respectively, have secondary effects that can be activated by removing the Ritual Spell Card from the Graveyard. "Ritual of Grace" prevents a Ritual Monster from being targeted by a card effect for a turn, while "Ritual of Destruction" empowers a Ritual Monster to return a monster to the top of its owner's deck rather than sending it to the Graveyard when destroying it in battle for a turn.
As of "Duel Terminal - Raid of the Inverz!!", Ritual Monsters have the support of an entire archetype, the Gishki. While most of this support is specific to their own "Evigishki" Ritual Monsters, one member of the archetype, Gishki Chain, can support any Ritual deck just as easily.
Example