Witches and wizards need training to learn how to control their magic. With young and untrained children, magic will manifest itself subconsciously in moments of strong apprehension, fear, anger and sadness. For example, Harry Potter once made his hair grow back after a bad haircut, set a boa constrictor on his cousin Dudley at the London Zoo, and made Aunt Marge inflate to an enormous size. While this reaction is usually uncontrollable, as an untrained child, Lord Voldemort was able to make things move without touching them, make animals do what he wanted without training them, make "bad things happen to people" who annoyed him, or make them hurt if he wanted to. In addition, Lily Potter was able to guide and control the blooming of a flower by wanting to.
Almost all magic is done with the use of a wand. On the subject of wandless magic, Rowling says:
||You can do unfocused and uncontrolled magic without a wand (for instance when Harry blows up Aunt Marge) but to do really good spells, yes, you need a wand.
A wizard or witch is only at their best when using their own wand. When using another's wand, one's spells are not as strong as they normally would be.[HP1]
Within the books, technical details of magic are obscure. Of Harry's lessons, only those involving magical creatures, potions or divination are given in any detail.
Severus Snape once told Harry Potter that "Time and space matter in magic" during Harry's first Occlumency lesson in Harry Potter and the Order of the Phoenix and Albus Dumbledore told Harry after finding the magically concealed boat to reach the locket Horcrux that "Magic always leaves traces, sometimes very distinctive traces."[HP6]
Spells are the every-purpose tools of a wizard or witch; short bursts of magic used to accomplish single specialised tasks such as opening locks or creating fire. Typically casting requires an incantation, most often in a modified form of Latin (see Dog Latin), and gesturing with a wand. However, Rowling has revealed that particularly talented wizards can cast spells without the aid of wand, although magic produced with one is generally more precise and powerful. Spells can also be cast non-verbally, but with a magical wand. This special technique is taught in the sixth year of study at Hogwarts and requires the caster to concentrate on the incantation. Some spells (e.g. Levicorpus) are apparently designed to be used non-verbally. While most magic shown in the books requires the caster to use their voice, some do not (and this may depend on the witch or wizard). Dumbledore has been known to do impressive feats of magic without speaking, such as conjuring enough squashy purple sleeping bags to accommodate the entire student population[HP3] or during his duel with Voldemort towards the end of Order of the Phoenix.
It is possible to use a wand without holding it. In Order of the Phoenix, Harry himself performs Lumos to light his wand when it is lying on the ground somewhere near him.[HP5] Additionally, Animagi and Metamorphmagi do not need wands to undergo their transformations.
Spells are divided into rough categories, such as "charms", "curses", "hexes", or "jinxes". Although offensive and potentially dangerous curses exist in number, three are considered usable only for great evil, which earns them the special classification of "Unforgivable Curses".