the poses are (can be) different. I did a regular yoga class a week ago and was pretty uncomfortable for most of it...not sure exactly what I was and was not supposed to be doing, and I hated being the only one doing something different. I did a prenatal yoga dvd over the weekend and LOVED it! Everything I could/should do and it was specifically designed to open hips, stretch, etc. in a way that was comfortable for me.
Re: prenatal v regular yoga
Prenatal yoga has specific poses for your body during pregnancy and poses/exercises to help during labor
Prenatal has more modifications for comfort and ease, but there are some poses that you shouldn't do while pregnant (backbends.... forwards bends).
I can't remember, but when I was practicing in the studio the instructor was insistant that she know if you were pregnant.
I find that I like to do a regular yoga session, but know my limits and modify the pose as I need it.