Woods witch

A Woods witch (often called just a "witch") is a type of woman in Westeros who practices herb-lore and healing. They also sell various charms, and some act as fortune tellers.

Whether or not woods witches actually possess real "magic" powers is debatable.

Known Woods witches

 * Maggy - a woods witch who lived in the forests outside of Casterly Rock when Cersei Lannister was a teenager. Young Cersei visited Maggy and demanded that she read her fortunes.  Maggy let Cersei ask three questions, and she foretold that Cersei would marry the king, and she would be queen some day, but only until cast down by another, younger queen.  Maggy also warned that Cersei would have three children, but all three would predecease her.

In the books
In the A Song of Ice and Fire novels, woods witches are most often not actually capable of practicing "magic", but have some decent knowledge of herb-lore and midwifery. This knowledge is apparently passed from woman to woman and through folklore. Their meager skills are scant compared to the systematic knowledge and training of the all-male Order of Maesters, who scoff at their claims to magical powers (though they also claim that "magic" doesn't exist at all). They are called "woods witches" because they often set up shop in the woods outside of villages, to sell various medicinal herbs and charms.

Woods witches do have reasonably effective knowledge of medicinal herbs. Asha Greyjoy (renamed Yara Greyjoy in the TV series) was taught how to make moon tea by a woods witch in the Iron Islands - moon tea is a common type of abortion drug, which maesters also know how to make. They seem to be called "woods witches" even when they are not actually living in woods, given that there are no major forests in the Iron Islands.

Qyburn, who is more inclined to believe in magic than many maesters, still says that most woods witches are basically amateur apothecaries without magical skill. However, he recognizes that the name "Maggy" that people called the woods witch that Cersei met was probably a slurred mishearing of "Maegi" - a female practitioner of Blood magic from Essos, and Cersei does recall that "Maggy" was a foreigner. If this was true, Maggy was not merely a regular woods witch. Throughout history, there are multiple instances in which it is remembered that a woods witch prophesied some major event - though the counterargument is that people just tend to remember the few times that their fortune telling happened to coincidentally be correct, and to forget the far larger number of prophesies which never come true.

It is said that a woods witch once prophesied that The Prince That Was Promised would be born from the line of House Targaryen (a description which actually matches Stannis Baratheon, because his grandmother was a Targaryen).