"Where do babies come from?"

I have read the wikipedia page on Montessori education and I feel like I don't fully "grok" it -- I'm not sure I see how it differs from conventional preschool except in seemingly minor ways (e.g. mixed age groups), but that's probably due to lack of experience with preschools.

Can anyone give me a better sense of what "Montessori" really means, how it differs from conventional preschool, and if there's any evidence that it's superior? Are there other philosophies like Montessori, or are preschools basically Montessori or not-Montessori?

Last question unrelated to Montessori -- is there any research showing the best time to enroll kids in preschool? I know there are some around me that will take them as young as 18 months, which is coming up quickly for my daughter. I'd be happy to move her but I feel like we are finally in a good place with her current daycare -- she knows and likes her teachers, she's on a good schedule, she has "friends" there (to the extent a 1yo has friends), I like the other parents, etc. So is there an advantage to moving her to a preschool at 18 months or should I wait until she's older? How old?