Fascias are traditionally a softwood board which is mounted to the exposed edges of your rafters, however in recent years this has been replaced with high grade uPVC as softwood has a tendency to rot.
The purpose of a fascia is to protect and seal the roof and the interior of your home from exposure to the elements, to support roof tiles at an eaves level and to provide support for the guttering.
Fascias also serve a cosmetic purpose, and are much nicer to look at than exposed rafters.
Modern options come in a range of styles and colours, bargeboards can be highly decorative and even flat fascias have options with designs carved and grooved into them