Spellcraft is the ability to cast and work spells. It is often used to describe the rituals performed by Wiccans. Such instances of spellcraft can require the use of various gems, herbs, incense, candles, colours, meditation, chanting, astrology, and numerology to produce the desired effect, which is typically beneficial or benign in nature.

Not everyone will agree on the definition above so I will add my two cents as well.

Spellcraft for Wiccans is causing a change or manifestation by connecting with Deity and focusing your attention on your intention. This is done many ways, using candle magick, talismans, amulets or very complex spells as in Ceremonial Magick. No matter what method of spellcraft you choose to use, the basic elements are the same. The tools used do not have any power on their own; they are used to aid in focusing your attention and some for holding or directing energy. The magick is the changes made to the Universe on the sub-atomic level by our thoughts, desires and actions. It’s magick simply because we don’t know exactly how it works; only that it does work.

Christians use Prayer rather than magick, but it’s basically the same save for the fact that they don’t take responsibility for the outcome. It is the will of God alone.