Montessori is just a type of daycare, following the educational approach of Maria Montessori. A 'true' Montessori school (any daycare can call itself Montessori but that doesn't necessarily mean they are) typically follows a very structured day, and using Montessori materials (you'll hear things mentioned like 'the broad stairs', 'the pink tower' which are sort of games really, but which teach mathematical concepts like widest to narrowest etc). It's a mixed age group classroom (usually 3-7).
There's loads more info about it here:
http://en.wikipedia.org/wiki/Montessori_education#Montessori_Education_TheoryBasically, it's just another form of daycare. Stan goes to a Montessori school and is absolutely thriving there - I think he likes the relative quiet and order, and likes knowing what happens when and where.