If you go looking for answers about what a doula is, you usually run across several variations that talk about a doula coming from ancient Greek meaning either slave or a woman who serves. Some places like to steer clear of the slave moniker, because over the obviously tainted connotations that go along with it, but the second definition doesn't quite go far enough in explaining the role of a doula either.
So what is a doula? In the context of home birth, a doula is someone, typically a woman, who is available to provide phyisical, intellectual, and emotional support to a mother during preganancy and child birth. There are also doulas who specialize in postpartum support for mothers, but that's a topic for another day.
While the midwife's role at birth is largely focused on making sure the baby arrives safely into the world, a doula is there to be a person the expectant mother can rely on for support in all capacities. Piror to birth, a doula is someone the mother can rely on as a resource to answer questions about how her body is feeling and can help you and the future mother of your child put together a birth plan.
During birth, the doula does everything from rubbing the birthing mom's back to helping reassure her that her laboring process is normal to providing support during particularly strong contractions. An experienced doula sees many births over the course of the year, allowing her to be more aware of what's normal in the birth process. If there's ever a time when complications arise, the doula is someone who can help advocate for your wishes in the event of a transfer to a hospital or other unforseen events.
While some of the emotional and physical support provided by a doula sounds like it might supplant your role as dad in the birth process, that's definitely not the case. With everyone's role fairly clearly definied going into the day of birth, it's a huge relief to know there are other people to depend on, because even though you as dad aren't doing any of the laboring to bring the baby into the world, the experience is emotionally overwhelming and at times physically exhausting. The birth day may arrive and find you more overwhelmed than you expected, which is totally normal. Having a doula there to assist eliminates many of the stresses you may be feeling as well as the stresses that may arise between you and mom.
We did not have a doula at my daughter's birth, but we did have plenty of support. My mother-in-law was on hand to provide assistance, and our midwife had two midwives in training. While any assistance a midwife brings with her is typically counted on to assist the midwife, in our case, there was definitely overlapping of duties where the midwives in training were fulfilling some of the role typically associated with a doula.