My son went to a preschool run by our school district and the biggest difference is the curriculum. It coincides with what they will be learning in Kindergarten and prepares them for the Kindergarten classroom. Regular daycares have their own curriculum that does not necessarily coincide witht the school district. Also, daycares don't have to have a curriculum if they are in a home environment, versus a center. My oldest is now in Kinder and he went to a home first, then preschool, now Kinder and I think it really helped with the adjustment, since it not only teaches the educational aspect of school, but also socialization, having to sit and listen, following directions, etc. I highly recommend preschool for every child, even if it's just a few hours/day. It really does make a huge difference in preparing them for school.