I have answered all the typical responses like:
"Doulas help to keep moms comfortable..."
"We provide physical support, emotional support, and knowledge to the birthing couple..."
"I am there to help the couple, not just mom, to have a great birth experience..."
But then the other day I answered it like this...

"When most people plan to purchase a home, especially their first home, they turn to the help and expertise of a real estate agent. They look around, take personal refferals and finally pick the agent that is the best match for them. This can be a personalty match, or maybe they like the way the look on paper, or maybe they use them because it is the same person their sister used. This agent works sits down with them, ask them what they are looking for in a home, what is most important for them, how the agent can be of best assistance. Then, she goes to work, providing them with information, pros and cons, and then s/he will walk them through the process from start to finish. Once the home buyer/s have found the perfect place, their agent then walks them through the paper work, the inspection, etc. Step by step their trusted friend helps them to get the best experience they can. If there are unexpected things that come up, s/he knows how best to navigate around them. After the purchase is complete, the agent usually follows up to make sure things are going well and to answer and questions they might have once they are in the home. This friendship they created will last a life time. When the buyer is ready to upgrade to a bigger home, they will once again rely on the trust and expertise that their agent provides.
Yes, this is very similar to a doula.

I truly believe that my real estate agent was incredible. She was patient and kind, she worked hard for us, and found us the home that we love! My doulas, yes I had two with my second birth, were just the same. They were incredible, they were patient (my second birth was 58 hours) they were kind and loving, and they helped me to have an incredible birth that I look back on with joy.
I have been to may births, and each one is personal to me. I become friends with each family, we create a relationship that lasts years. I wouldn't change it for the world, I value each family and the joy I get from helping them through the birth process.
And who knows, maybe one day I will get to be the doula for my real estate agent... that'd be pretty cool :)
(realestateimage from www.successfulthings.com)
(doula image from birthdoulasofpittsburgh.com)