On the surface the terms "spell" and "ritual" seem interchangeable meaning more or less the same thing. In grimoires, books of shadows, and other sources of esoteric study, these two words are commonly confused.
In truth, these words have very different definitions with the word "spell" (also defined as a "state of awareness", "state of being" or "state of mind") implying an action that is "short and capable of manifesting some kind of change".
The word "ritual" in a magickal sense implies an action that is "repeated to create or regain" a certain "state of mind". This sounds like the same thing, but rituals are often performed before and after spells to achieve the appropriate atmosphere in either the mind of the witch, the witch’s natural environment (physical plane) or in the astral plane where spirits dwell.
Subsequently, the most significant difference between spells and rituals is that rituals have been historically designed for purposes of maintenance and preparation . Spells, on the other hand, are most often intended for the manifestation of some immediate or unique change (or the prevention of change).
Typically, the time it takes to perform a spell is short and sharply goal-oriented. A ritual, however, can last anywhere from a few seconds to several years. In fact a single ritual, can have several spells for several different aims included within it. A spell should have a single purpose while a ritual may have many. Subsequently, many spells are preceded by and followed by rituals for summoning, banishing, purifying and cleansing.
Rituals are also used to keep individuals in touch with the changing of the seasons and are often performed to welcome and celebrate the Sabbats of the pagan calendar. Likewise rituals as dedications to deities and celebrations of life stages (maturity, croning, handfasting, birth, death, etc.) are quite common in pagan culture and need not involve the change of anything.
Communing with spirits through invocation or evocation is another common use of ritual work. Witches and magicians can grow with ritual work and become more highly evolved without ever effecting change in their environments. Ritual work is necessary for performing acts of High Magick (the act of causing the soul to unite with or rise to meet the universal deity, god, goddess, higher self or universe, however you see it).
Some of the similarities between spells and rituals include that fact that both types of workings can involve all three kinds of magick: Natural, Spiritual, and Mental Magick. A spell however, must include at least ONE of the three types to be considered a working of spellcraft, because at the end of a spell, something is expected to manifest. We expect the world to be somewhat different than it would have been had we not performed the spell.
A ritual, on the other hand, does not necessarily require one of the three forms of magick because the ritual itself can simply be done to honor or celebrate an individual or time period. Rituals workings are common for initiations as well as greeting newly born children and bidding farewell to those who have passed on. Rituals are repeated. We come to expect them through out our lives as they help us mark important occasions and connect with nature.
Spells, on the other hand, are performed more by necessity than habit. We may cast a spell to heal an illness in a loved one whereas we might perform a ritual once a year to ensure stronger health throughout the coming seasons.
Both spells and rituals can both be considered workings of witchcraft or magick, though rituals are common even in mainstream religions (Christianity, Judaism, Islam, etc.). Conversely, the practice of casting spells is not shared by mainstream religions and many devoted members of these faiths fear the three forms of magick (even though they perform Spiritual Magick regularly in their own religions though with a different deity than most witches).
As you can see the line separating spells and rituals is very thin, and there are often times when the two categories will overlap. And, while it is not necessary to label each working you perform, having a general understanding of the differences will greatly assist in the understanding of the three forms of magick and how they can best assist you in everyday life.