A Doula's will become a mother's birthing partner, for whatever reason you require one. A Doula will give advice and help with the labour process, breastfeeding, calm your fears, encourage you and some will even help with housework and cooking in order to free the mother to care for herself and the baby.
It is a Doula's job to concentrate on the mother's needs, not the baby's. They will not normally be medically qualified, although some are.
They are there to help voice the mother's opinions and to help reassure you that everything is ok, before during and after the birth. Some may only be available for a consultation with the mother before and after the labour, then be on hand for the labour. Whereas other's may be available much more in the weeks leading up to the birth and after either on a full time or part time basis, depending on your requirements.