A doula is a person, usually a woman, who is professionally trained to assist a woman during childbirth and who may provide support to the family after the baby is born.
So, I know you're thinking to yourself "But, my husband/mom/sister/best friend is going to be there and that's all I need!" Trust me, I've heard that concern before. I actually thought the exact same thing when I was preparing for my first birth years ago, and followed through with only having my husband there for support. So WHY do I think women need doulas? Well, because sometimes you need more than just one person. Sometimes you need someone who isn't emotionally invested in you and your baby (not saying the doula ISN'T, but it's a different kind of emotional investment!). Sometimes, you need someone who has 'been there, done that' to tell you that everything is normal and you're doing fine.
Doulas are not your husband. They aren't your mom, they aren't your sister, they aren't your best friend. What they are, though, is priceless. If you don't want pain medication, the doula will help you get through transition by reminding you that you're so close to meeting your baby. She won't see you in pain and want to fix it, because she knows that sometimes pain is a good thing and if you're asking for the epidural, you're probably pretty close to done. If you do want pain medication, she'll make sure that you understand what, exactly, the medication used is - the risks, the benefits, the side effects - so you can make an informed decision. If you order pain medication but the anesthesiologist is helping someone else and you have to wait for a bit (it happens!), the doula will help you get through that waiting period. If your husband/mom/sister/best friend IS the best support person for you (they do exist!), the doula helps them help you. Need ice chips? Done! Need a cool washcloth? Done! When your support person just can't leave you, the doula is there to be their extra set of hands.