Here's a particularly relevant excerpt:
"All pre-K programs have three characteristics in common. They are (1) governed by high program standards, (2) serve 4-year-olds or sometimes both 3- and 4-year–olds, and (3) focus on school readiness."
Preschools on the other hand vary in their approach and focus, and serve a wider age range of children. For instance, our daughter attended a play-based preschool co-op for ages 3-5, which didn't focus on kindergarten academic standards. However, it did include activities that helped with reading, math and science skills, and our daughter was very well prepared for kindergarten (she succeeded that year in school too). Some preschools do focus on academics, and others offer a combination of play and academics.
Below are some additional resources that you may find helpful to your decision making process, as well as for this stage of your child's development and education.