A postpartum doula supports whole families through the fourth trimester, and beyond, by providing evidenced based information on a wide variety of perinatal topics. These can include:  infant feeding, infant sleep, emotional and physical recovery from birth, bonding between baby and each member of the family, infant soothing, basic newborn care, household organization and light cleaning, as well as preparing meals. Part of the postpartum doula role is to "mother the mother", making sure she is well fed, hydrated, and comfortable, and to teach partners and siblings to do the same, to create an integrated network of support that can sustain the family as the newborn grows.

For a more in-depth description, I recommend this article, by Emily Flynn, from Natural Child Magazine: Why Hire a Postpartum Doula?

As Americans, we are under the impression that new moms are “Superwomen” and can return to life as it was before baby. We must remember to celebrate this new mother and emulate the other cultures that honor new mothers by caring for them, supporting them, and placing value on the magnificent transformation she is going through. This is the greatest gift we can give to new mothers and newborns. A postpartum doula can help bridge the gap of support for new mothers and babies by providing the support and education they need to become confident new mothers.
—Darla Burns, CAPPA Executive Director of Postpartum Doula Programs

Postpartum doulas can provide a general uplift in your postpartum experience; included below are some specific benefits of doula support:

• Birth mothers who are relaxed, and nurtured through doula support, experience a boost in oxytocin release, which increases milk production.
• Doula supported fathers and partners learn skills earlier and are sooner able to take on solo infant care.
• Grandparents have an opportunity to learn new information about newborn care from doulas, as well as techniques for supporting new parents and their choices.
• Siblings benefit from the extra attention, either from the doula or the supported parents, and learn to better integrate the newborn into their family.
• Doulas provide resources and evidence based information about baby gear, and can help families acquire what they need, and no more. 
• Doulas facilitate clear communication between infants and their families by providing education on newborn cues, including sleep, hunger, alertness, and discomfort, helping to reduce confusion and increase understanding and the meeting of baby's needs.
• Doulas are trained to recognize normal hormonal mood shifts and their possible change into postpartum mood and anxiety disorders, advocate for early intervention, and offer non-judgemental emotional support and referrals to vetted providers who are able to diagnose and treat. 
• Precious time and energy can be saved by learning from a doula's experience with finding trusted, reliable options for information on newborn care and parenthood, including videos, books, web sites, and community resources.