A spell is a magical action often accompanied by an incantation that makes use of magic in order to accomplish feats ordinary people cannot perform, for example levitating objects, conjuring weather, or turning a person into an animal.
To cast spells, you must have complete control of your voice because the slightest change in decibel level (that's the sound of your voice) can change a spell's result. When Mildred's summer drink overflowed onto her desk, her voice was too loud. This is why chanting is so important, because it builds the strength of the voice and diaphragm (a muscle in your stomach, which helps you breathe).
In 1998 TV Series, Weirdsister College (TV Series), and The New Worst Witch, spells have pseudo-Latin incantations, while the 1986 Film and the 2017 TV Series use English incantations. Spells are mentioned in the books, but no specific incantations are given.
As witches become more experienced with spells, the words do not matter so much, and you can even make up your own spells by stringing words together- though they may not always work! With enough experience, a witch can cast spells non-verbally (without speaking). Miss Hardbroom and Miss Cackle regularly use non-verbal spells, while Mildred and her classmates are seen using them in their third year in the 1998 TV series. Spells are often accompanied by hand gestures; Miss Hardbroom often uses finger sparks as well.
For lists of spells, see List of Spells in The Worst Witch (1998 TV Series), Magic in Weirdsister College, List of Spells in The New Worst Witch, and List of Spells in The Worst Witch (2017 TV Series)
The Foster's Effect
The The Foster's Effect can happen when casting too many spells. With all that energy flying around at the same time, magical paths cross over and cause something known the The Foster's Effect. This reaction causes spells to multiply out of control, and has created all sorts of natural disasters. Using magic for selfish and trivial ends can also cause the The Foster's Effect. The Foster's Effect was discovered by Dr. Foster.
Transformation spells Transform an object, person or animal into something different. Don't try this unless you know a reversing spell, because the effects are irreversible without one. Such spells include those for transforming yourself or others into animals, and for transforming animals or objects into something else.
A reversing spell returns an object, person or animal to its natural state, after being affected by another spell. A simple reversing spell will fix most things. In the 1998 series, a common reversing spell is used to undo most other spells.
A moving spell will create a particular visible action, which will cause an object, person or animal to move away from its original point, e.g. moving, pushing, pulling, summoning, throwing, levitating. Moving spells are simple, and are some of the first magic that new witches learn.
A transportation spell allows one to disappear effortlessly from one place and end up in another. Not to be confused with a moving spell, where everything remains visible. When first learning transportation spells it can be diffcult to get the destination right, which can lead to some entertaining situations.
In the books and 2017 TV series, these are called Transference spells, and it is this spell that Miss Hardbroom uses to materialise from place to place, and she sometimes uses these spells on the students as well.
Appearing and Disappearing Spells
Unlike a transportation spell, which moves somthing from one spot to another, appearing and disappearing spells make things appear or disappear completly. They are advanced spells, with appearing spells being more difficult than disappearing ones. These spells are often seen being done non-verbally by the more powerful witches (such as Miss Hardbroom confiscating things). It is apparently possible use an appearing spell to bring back somthing you vanished with a disappearing spell. In the 1998 TV series, the art of vanishing somthing is called 'puffication' and bringing somthing back is called 'repuffication'.
A continuance spell causes the looping of an action and causes it to go on forever, or at least until it is magicly stopped.
Restoring spells are used to restore objects back to their former state, or to fix things. Not to be confused with a reversing spell, which undoes another spell.
Spells that affect the weather, such as creating wind or snow.
There are many other spells that don't in the above categories.