The word doula is a Greek word for slave or servant.  Doulas provide emotional and physical support during pregnancy, labour, birth and sometimes postpartum.  This is the most common term you will hear, however, sometimes individuals will also call themselves many things including Childbirth Assistant, Labour Support Professional, Birth Assistant or Birth Companion to name a few.

A doula does not speak for a couple, provide medical or clinical skills or act in any unprofessional manner.  In fact, most organizations have policies, a code of ethics and a standard of practice that each doula must follow.

explanations of medical procedures
emotional support
advise during pregnancy
exercise and physical suggestions to make pregnancy more comfortable
help with the preparation of a birth plan
massage and other non-pharmacological pain relief measures
positioning suggestions during labour and birth
helps support the partner so that they can love and encourage the labouring woman
avoid unnecessary interventions
written record of the birth
many other possibilities that vary from doula to doula