Rituals are highly complicated spells.
They are done when a wizard can't summon the required focus needed for a spell (magic). The magic doesn't come out of the person doing the ritual, it comes from an otherworldly force that is tapped into.
Harry uses the metaphor of a cosmic vending machine: an outside power offers to give you something if you fulfill a certain sequence of events. Another metaphor he uses is: you have to load the gun, aim it and pull the trigger.
If a practitioner has any real skill of their own at magic, they don't need a ritual. 
Wizards use rituals because they are unable to focus on all the parts of a needed spell. It makes the spell easier to do. Rituals can be as basic as making a magic circle to a Sanctum Invocation. High level rituals are extremely complicated and difficult to get right. The variables in a ritual could include the direction a certain candle is pointing, materials that have certain meaning to the wizard, ties to certain spiritual concepts like the four elements or the mind body and spirit, any other magic tied to the wizard.
Some examples of materials that could be used include: a crystal (representing earth), pictures of loved ones (representing heart), hair/blood/etc... (for when directing a spell towards a specific person or group of people), etc..
Rituals have a variety of purposes, the most common being tracking people down, summoning, and trapping individuals.
Dresden's opinion of ritual magicEdit
It's not Harry's favorite form of magic. He feel sort of silly and self-conscious bathing, dressing in a white robe with a hood, lighting candles and incense, chanting, and "mucking around with a small arsenal of candles, wands, rods, liquids, and other props" normally used in Ritual magic. He also sees an advantage that the props and process hold for him by freeing up his mind from having to imagine the little details when it comes to heavy duty magic. Normally it's second nature and fine for short term magic. For longer spells he needs a far greater level of concentration.
In the series Edit
- Main article: Storm Front
In Storm Front, the primary antagonist, the Shadowman, uses a ritual to make his victims hearts explode. The ritual itself is a dark and powerful form of Thaumaturgy, using lust generated by a man and women having sex and channeling the power of a thunderstorm to power it out. The Shadowman would then take a rabbit with a piece of his target tied to its back and remove its heart with the edge of a sharpened spoon.
- Main article: Blood Rites
In Blood Rites, the three ex-wives of Arturo Genosa use ritual magic using an entropy curse to kill off the future Mrs. Genosa by targeting all the women around Arturo, one by one. Lord Raith is behind it all, pulling their strings. Raith also attempted at the end to sacrifice Thomas to kill Harry with an entropy curse in order to end Margaret LeFay's death curse on him, which weakens him, by killing both of her sons to undo it.
- Main article: Proven Guilty
- Main article: Turn Coat
- Circle of Power
- Stone Table
- Medea's bodkin
- Objects of faith
- Jake Guffie
- Arturo Genosa
- Madge Shelly
- Lord Raith
- Thomas Raith
- Trixie Vixen
- Victor Sells
- Arianna Ortega
- Red King
- Chichén Itzá