What is a Ritual?
A ritual is a magical effect similar to a spell. A character that has learned to "trace" rituals can do so at any time; after tracing, the ritual remains "charged" until the tracer decides to discharge it with an act of will, at which point the ritual takes effect. Discharging a ritual requires a free action (unless otherwise specified) which can only be performed on your turn, but you can take any number of such free actions, up to and including discharging all your charged rituals at once. In this way, a ritual tracer can store up ritual charges until they are needed. A ritual that is not discharged within 24 hours of its charging fades away with no effect. The duration of a ritual's effect depends on the method by which the tracer learned the ritual; see the individual class and feat descriptions for details.
Rituals come in five grades: minor, least, lesser, greater, and advanced. Tracing any ritual requires a diagram to be drawn on a flat, solid surface with a piece of chalk or charcoal; it also requires a drop of the tracer's blood (which deals 1 point of damage), and chanted words (a ritual cannot be traced while under the effect of a silence spell, for example). The length of time it takes to trace a ritual depends on its strength: a minor ritual takes 1 minute, a least ritual takes 5 minutes, a lesser ritual takes 10 minutes, a greater ritual takes 15 minutes, and an advanced ritual takes 30 minutes to trace.
Rituals are supernatural abilities. They cannot be counterspelled and are not subject to Spell Resistance, but they are magical effects and can be dispelled, and are suppressed in an antimagic field or dead magic zone (charged rituals remain charged, but cannot be discharged in such areas).
Charged rituals take different forms for different tracers, depending on how they were taught the arcane formulae. Some ritual tracers imbue the magical energy into concoctions that they imbibe, infusing their bodies with the charges; others draw sigils on their skin with specially prepared inks. Some ritualists have no outward evidence of their charged rituals, instead drawing the charge directly into their essence, where perhaps the augmentation is visible in their aura.
If a character has the ability to have more than one ritual charged at a time, she may charge the same ritual more than once. The number of times a given ritual can be traced per day varies; generally, rituals gained from a ritual class can be traced any number of times per day, while rituals learned from feats can only be traced once per day. A character who learns to trace rituals from feats (instead of class levels) may have only one ritual charged at any time unless they take the Extra Charge feat.
Stacking of Rituals
Like spells and similar effects, the bonuses granted by rituals generally don't stack. In particular, multiple instances of the same ritual do not stack with each other; different strengths of a ritual count as the same ritual for determining if a particular effect stacks.