Charms are using Mahou or magic in order to alter an object's properties, or give it new properties. However, that isn't really an absolute definition. A patronus charm does not alter an object's properties, it is instead a conjuration of pure happiness. In addition protection charms can conjure shields and other defenses. In a more general explanation, charms usually are spells which are inherently 'positive' and harmless rather than 'neutral' like Transfiguration or 'negative' like the dark arts. For example tickling charms, protection charms, the love charm, etc. While Charms alter an object's properties, Transfiguration spells can change the object into something else, vanish it, or even conjure a new object. Charms just change the object, summoning it, changing its color, etc. When an object is charmed, it's called 'bewitched', and charms usually use more extravagant wand movements, and more elaborate invocations than say a Transfiguration spell. Charms are shot as a beam of magic. Charms can also be put to destructive effect however, since it can change an object's property to 'destroyed'. All are within the realm of possibility, and the only limitation to a charm is your own imagination.