For as long as Hogwarts has existed (and likely before) Magic has been separated into two distinct categories, Light and Dark. These become broken down again and again, until we get Charms, Transfiguration, Defensive Magic, and then again into spells of their own, but first and foremost magic is split into Light and Dark.

Now most spells we use are old, well known and already categorised and most new spells go into the same pot as the wix who created it. A Dark Witch creates a spell, it gets categorised as Dark, a Light Wizard creates a spell, it goes into the Light pot. This isn't necessarily right, as any wixen can create any kind of spell, regardless of their own orientation and ignores the fundamental differences that separate Light and Dark Magic. 

Now most of this problem stems from the fact that most people don't know what these differences are, and how exactly one identifies a Light Spell, from a Dark Spell. Another problem is our own Ministry of Magic, who in recent (and not so recent years, see the overhaul of 1654) years have changed classification according to their beliefs or to suit the needs of the time. The final reason most people can't tell a Light Spell from a Dark Spell is the connotations that 'Light' and 'Dark' have on them now. Most would assume a Light Spell is a good spell, whereas a Dark Spell is an evil one, and this simply isn't the case. Magic is not defined by good and evil, though the intent in a spell can shape it, and there of course areas of grey. Light and Dark magic are simply different, and the concepts of good and evil do not define them.

If you cannot realise and accept this (and I know it may be hard) then this book isn't for you. Many of your views will be challenged and I would like you to understand that before we delve into the realm of magic.

Okay? Then move on.