Doulas are trained to assist and support a woman during labor and childbirth. They're not nurses or midwives, so they don't help with the actual birth. But they provide physical and emotional comfort, everything from chatting with you to holding you while you cry to massaging you.
Susan Spicer of Canoe.ca reported on a study showing that a friend or family member can successfully stand in for a doula after just four hours of training. The study of 600 women found that having a lay doula significantly shortened labor.
Spicer points out that getting emotional support from someone likely helps you release oxytocin, the hormone of love and connection and also the hormone that speeds childbirth.
So, even if you don't plan to have a midwife or doula, you might want to have a designated friend. I imagine the training focuses on how to keep the laboring woman -- and yourself -- calm.