From Discworld & Terry Pratchett Wiki
Like psychology, but many witches think "psychology" is a bad word, or that it means "having a psychological problem". The practice of headology relies on the principle that what people believe is what is real. This is used by witches to earn respect or at least fear, and also to cure patients. For an exemplary combinatorial use of headology, herbal medicine, and physical therapy, see old Jarge Weaver's visit to Granny Weatherwax in Maskerade.
The power of Headology is not to be underestimated. Clearly, the way a person sees himself and the surrounding world forms the person's reality. If this view is changed effectively through the use of Headology then this person's reality changes. This allows witches to make people think they are frogs, for example. Witches generally think that Headology is a more powerful style of magic than any of the fancy stuff wizards use. Headology is what witching is all about. A witch needs a very powerful, focused, and trained mind to use Headology. Granny Weatherwax is the foremost practitioner of Headology on Discworld.
Headology can take advantage of a voodoo witch's belief that anything done to a doll is also done to the person that doll represents, and turn it back on the person holding the doll (so as to stick pins in it) in some surprising but logical ways. It can leave a family of vampires, who have just ill-advisedly dined on Granny's blood, incapacitated and craving nothing more than a very strong cup of tea with six sugars in it: Granny may have been vampired, but the vampires discover that they have also been well and truly Weatherwaxed.
Power of Accessories
Many kinds of accessories confer a certain degree of identity. For example, The Archchancellor's hat is the symbol of magic under the control of wizardry, and each wizards' hat signifies them as a wizard to non-mages. A warrior's helmet confers the identity of a warrior (see Lords and Ladies). A bone-white mask confers the identity of a mysterious artist (see Maskerade). And a witch's hat confers the authority of a witch. Generally, a witch is most powerful when the people around know (or at least think) that she is a powerful witch. The witch's hat is a symbol of office. Younger witches often feel much better for having their hats on, some even wearing it indoors.
Similarly, the Postmaster's golden wingèd hat is a powerful thing and confers respect on all who see it, as well as a sense of power and responsibility on all who wear it.
See also: Boffo