So What is a Witch?

So what is a Witch and why have we been writing about them for centuries? This is a definition taken from Wikipedia:

Witchcraft (also called witchery or spellcraft) broadly means the practice of, and belief in, magical skills and abilities that are able to be exercised individually, by designated social groups, or by persons with the necessary esoteric secret knowledge. Witchcraft is a complex concept that varies culturally and societally, therefore it is difficult to define with precision[1] and cross-cultural assumptions about the meaning or significance of the term should be applied with caution. Witchcraft often occupies a religious, divinatory, or medicinal role,[2]and is often present within societies and groups whose cultural framework includes a magical world view.[1] Although witchcraft can often share common ground with related concepts such as sorcery, the paranormal, magic, superstition, necromancy, possession, shamanism,healing, spiritualism, nature worship, and the occult, it is usually seen as distinct from these when examined by sociologists and anthropologists.

Real life witches are individuals both male and female who revere nature and practice spell casting and healing. There are many  types of witch (hedge witch, kitchen witch, etc) and each one uses different tools and have different ways of working their craft. Common items used are crystals, candles, herbs and tools such as athames and even wands. Spell casting is done to aid in healing, attracting money, love and more. 

The Wiccan Rede:

Hear now the word of the Witches, the secrets we hid in the night, 
When dark was our destinys pathway, That now we bring forth in the light.

Mysterious Water and Fire, The Earth and the wide-ranging Air, 
By hidden Quintessence we know Them, and we will keep silent and dare.

The birth and rebirth of all Nature, the passing of Winter and Spring, 
We share with the life Universal, rejoice in the Magical Ring

Four times in the year the Great Sabbat, returns, and the Witches are seen, 
At Lammas and Candelas dancing, on May Eve and old Halloween

When daytime and nighttime are equal, when sun is at greatest and least, 
The four lesser Sabbats are summoned, again Witches gather in feast.

Thirteen silver moons in a year are, thirteen is the Covens array, 
Thirteen times at Esbat make merry, for each golden year and a day.

The power has passed down the ages, each time between woman and man
Each century unto the other, ere times and the ages began.

When drawn is the Magickal circle, by sword or athame of power, 
Its compass between two worlds lies, in the land of shades of that hour.

Our world has no right to know it, and the world beyond will tell naught, 
The oldest of Gods are invoked there, the great work of Magic is wrought.

For two are the mystical pillars, that stand at the gate of the shrine, 
And two are the powers of Nature, the forms and the forces divine.

And do what thou wilt be the challenge, so be it in love that harms none, 
For this is the only commandment, By Magick of old be it done.

Eight words the Witches Rede fulfill:
If it Harms none, Do what Thou Will!

The most important part being the last line, which means that you must never cause harm to another. If you do it will come back on you threefold.
Witches in fiction are obviously exaggerated and tend to have superhuman abilities, but many authors will base spell casting on actual rituals. Whether fictional or real, most witches are strong individuals who try to help others and promote healing. There are plenty of books and information available if you would like to learn more about Wicca and similar practices.