When I was a child school started at first grade. As more and more women got jobs and entered the working force there developed a greater need for school before first grade. How were mothers going to be able to go to work if they had to worry about what to do with their children who were not in school. The public school system was the first to start offering kindergarten. Many of the private schools started soon afterwards. Kindergarten then became part of school.
After people noticed the success of the kindergartens they realized that the children younger than kindergarten also needed to be in school while mom went to work. So private schools started offering school before kindergarten. To differentiate between the two grades they started calling it preschool. What happened in lots of other schools across the country happened though, is that other shcools starte calling any schooling before first grade preschool. So as of now for some people preschool is anything before first grade and for others it the grade before kidnergarten.
Its pretty much all the same though. You still need your creative curriculum lesson plans that you can find on kindergarten teacher websites. You still need to understand children and love children. The main difference in the two grades however, is what you can expect from the children on a developmental level. The kindergarten children are a whole year older than the preschool kids and it shows in many ways. It can be a dangerous thing if you start giving the younger children activities that are really for the older children and vice versa. The older children will get bored with activities that are more appropriate for younger children and the younger children will get frustrated if they get activities that are really for children a year older than them.