Witchcraft is the magical manipulation of energy to bring about change.
The energy used by a witch may be environmental, from herbs, stones and other natural objects, or it may come from the witch herself or it may be channeled through the witch from a God or Goddess or from the Earth or Universe at large.
Witches are best known for casting spells which tends to incorporate all of these methods in a ritualistic practice. However, one need not engage in ritual or cast a spells to perform magical acts or to be a witch.