In Welsh Folklore Who Are The Tylwyth Teg?

The Welsh name for fairies is y Tylwyth Teg meaning the fair folk or  fair family.
This is sometimes lengthened into y Tylwyth Teg yn y Coed, the fair family in the wood, or Tylwyth Teg y Mwn, the fair folk of the mine.
In appearance the Tylwyth Teg were said to be small in stature, with golden hair, a common trait in many fairies. They were thought to live under hollow hills and in deep crevices, and to frequent ancient places such as Bronze Age Barrows or cromlechs. The stories of fairy folk inhabiting ancient sites is common throughout Britain and Ireland and go back into the mists of time

In common with other  fairy folk they often interacted with mortals in the past, and it was possible for a mortal to marry a fairy , although they always longed to return to their own people and the endings weren't always happy ones..
They are spoken of as being kind with their blessings on those humans whom they select to be thus favoured. Because of this they are called Bendith y Mamau, or their mother's blessing.
It's not a good idea to name the fairies by a harsh epithet as this can make them angry. Speak to  them in flattering terms to ensure favourable treatment!