In terms of prenatal care, not much. A midwife is just as qualified as an OB to take care of you during your pregnancy. A midwife can also deliver your baby if you choose to have a natural birth with no medical intervention (pain medication, or c-section).
The main difference is a midwife cannot prescribe medication, or can't help if you need the before mentioned medical intervention. Midwives are perfect for low risk pregnancies. Most midwives work in a team with at least one OB, so if complications arise the transition to an OB is very smooth.
Personally, I love my midwife. I find her extremely friendly, and always has the time to answer my questions and address my concerns. Compassion. Something I have found many doctors lacking (in my experience).