Doulas - Who? What? Why?

I had heard of doulas before - but wasn't sure what they were or what they did. I got them confused with midwives and thought it was just someone only women having home births had to help them. Then, I came upon the term again in my reading, which I have done a lot of at this point.

So... what is a doula? A doula is trained to provide emotional and physical support to women during childbirth. Doulas do not perform medical tasks, unlike midwives - some of whom have nursing degrees.  Though I originally associated doulas with homebirths, some sources actually say that it's more important to have a doula for hospital births.