Authentic African masks are not pieces of wood more or less decorative, but religious symbols with an important role in the life of the village. Generally they are used in funeral rites and ceremonies related with agriculture or initation, in which each young man has to participate to be considered a member of the community. There are many ethnic groups that don't use masks, and many others that don't have rites of passage.
In funeral rites, the mask takes the vital strength thet runs away from a being when it dies. The mask control this vital strenght in order to avoid that it hurts someone, and distribute it in benefit of the community.
During the ritual dance, the porter is protected by the mask, that transforms him in another being. He must be dressed not to be recognised. The dress is also considered part of the mask, but the most important thing is the head, where the vital strength is.
Agriculture is the main occupation in rural zones. Rituals with masks use to be celebrated during the dry season, because the lack of work.
There are in Africa many different masks. Here you have a little selection of well carved reproductions from subsaharian west Africa, suitables as a good introduction to this amazing world: