A spell or incantation is a series of words that, when spoken, invoke some magical effect. Reciting a spell is commonly referred to as casting a spell or chanting. In order to be effective, spells must be performed by a magical being. To this end, a spell recited by a mortal will not work. Whilst spells require to be spoken they can also be performed non-verballyand can be channeled and used by powerful telepaths.
Spells that create negative magical effects, that harm others, are called curses. These spells are difficult and should not be taken lightly.
The ability to cast spells is called spell casting. It's a basic ability that all natural born witches possess (along with scrying and potion making). Some spells require more power and thus must be cast by multiple witches at once such as the three Halliwell sisters.