Classic Lolita is one of the three main Lolita styles (the other being Gothic and Sweet). It is characterized by a more historical, classy and lady like. The palette of Classic Lolita is muted nature colors like ivory, brown, beige, wine , navy and dusty pastels. Floral prints are common and prints featuring classical paintings. The theme is more inspired by historical clothing like Victorian and Rococo fashion. Due to classic being a more mature style many Lolitas turn to it as they get older. Natural haircolor and natural makeup is most popular. Lately Longer skirts have been gaining popularity which have led to discussion of when it stops being Lolita, though I think that new ideas are a good thing as long as the general aesthetic of Lolita fashion is kept. I think the full length dresses could be considered a form of Classic Aristocrat.
Some of the biggest Classic Lolita brands are Victorian Maiden, Innocent World, Juliet et Justine and Mary Magdalene.
I made a collage to help you get the general feel of the style visually:
None of these are any rules of the style, just descriptions of whats common so that you can get an idea of what the style looks like, so don't be afraid to experiment!