Preschool is a general term for early childhood education. The curriculum can vary widely depending on the school and the director's philosophy about early childhood education. Classes can also vary depending on the individual teacher, but there are some schools, where the curriculum is exactly the same no matter who the teacher is (of course, this does not take into account personality differences between teachers). Some schools really push academics, while others believe that playing and learning social and emotional skills along with age-appropriate academics are what is most important.
There is a difference, however, between early preschool classes and an actual pre-K class. Early classes are usually grouped into 2-3 year-olds; 3-4 year olds; and then Pre-K classes, which are usually 4-5 year-olds. Your child usually has to be a certain age by the school's cut-off date to be placed in a certain class (e.g., must be 3 by September 30 to enter the 3-4 year-old class. This can also vary by school or state).
In many cases, early preschool classes focus on getting the children used to being in group settings, sitting for circle time, sharing, playing appropriately, and teaching basic age-appropriate academics. Pre-K classes place even more focus on academics and the skills the children will need to be successful in kindergarten. Again, this can be quite different from school to school. For example, Montessori schools will be different from more traditionally-based preschools.
Although your daughter does not need preschool at this time, it is good that you are preparing and educating yourself on the available options. You might want to do a little research on early childhood education curricula, the different types of preschools, and ask yourself what is most important to you for your child. Also, ask yourself, "what does my child need?" and then make a few visits to the preschools in your area.
Sometimes parents, eager to ensure their children have every advantage, push the academics too hard and too fast while failing to realize that children also need to develop their social and emotional skills in order to be successful in school and in life. My son could easily say the alphabet and knew his numbers, colors, and shapes by the time he started preschool, but like most preschoolers, he needed help with sharing, delaying gratification, learning to cooperate and compromise, using words instead of hands, and dealing appropriately with strong emotions such as anger and frustration. Within his preschool setting (and in conjuction with what we teach him at home), he has been able to learn these important skills and has grown so much in the last two years.
I'm not one of those people who believe your child has to be reading by age 4 (although it's fine if they are) to be successful in life. I don't believe in pressuring children at such an early age. Learning should be fun at this point. You don't need preschool at this point to do that. As one of the other mothers mentioned, playdates at parks or home, classes at the gym or rec center, or mommy and me groups will all give her time to interact with other children and learn these valuable social skills.
Finally, when it does become time for you to choose a preschool, get a good feel for the people who are teaching. They should be mature, well-balanced people who love children and love what they do. For some, it is just a job, and in those cases, the children suffer. Your child should have a safe, warm, and positive experience as her introduction to the educational process. What happens there sets the stage for the next phase of her education.
Good luck in your search!