Witches are mortal beings that can access and manipulate magic.
In 212 B.C., a woman and her lover were able to tap into nature, and harness it's magic. The lovers had two children, one good, and one bad. The good child, a daughter, had eight children, and began a line of Pure witches. The evil child, a son, had five children, and began a line of Pure warlocks. Today, many wiccan lines decend from the Originals, including the Warren line.
Practitioners are mortals who can access and manipulate magic with the assistance of magical surrogates such as the elements, pentagrams, or crystals. Throughout multiple generations of practitioners, a magical child will be born, who will begin a line of magical witches.
Magical witches are mortals who can access and manipulate magic without the assistance of magical surrogates. All magical witches have supernatural powers. They can cast and invent spells, brew potions, and locate beings by scrying. They can also enchant and curse objects and people. They also usually have one or more personal powers. These powers can be used at will, and don't require the use of spells or potions. Personal powers seem to be heriditary, all the Halliwells have powers similar to their ancestors. Magical witches also seem to age slower than the normal human, demonstrated by all the Halliwell sisters, who appear to be in their late thirties, instead of their early fifties. They also have a slightly longer life span.
All witches seem to have a connection with one another. This is witnessed when Melinda could sense a witch at her school, she described the feeling as a chill in her spine.
Being human, witches are vulnerable to almost everything an average human being is vulnerable to. But with the ability to manipulate magic, they are often able to avoid death by using their powers.