A princess dress, not a costume
The difference is construction. A costume princess dress is usually unlined polyester with the decoration printed or glued on and nothing holding the skirt out. A Lolita dress has a lined bodice, shirring or lacing at the back so it fits, lace and ribbon sewn in, and is cut to be worn over a petticoat that gives the skirt its shape. It is made to be worn many times, to a party, a tea room or a museum, not once.
Sweet or classic
Sweet Lolita is the pastel, bow-heavy version: pink, cream and mint, playful prints, and a rounder bell silhouette. Classic Lolita is the quieter version: florals, jacquard and embroidery in wine, ivory and dusty rose, a softer A-line, and details borrowed from rococo and Victorian dress. Both are princess dresses; sweet reads as a fairytale, classic as a portrait. Hime Lolita sits at the ornate end of either, with crowns, bonnets and a lot of lace.
Where people actually wear them
Birthdays are the most common reason to buy a first one, followed by tea parties, photoshoots, conventions and weddings as a guest (in a colour that is not white). Plenty of wearers simply wear them out. A dress with a removable bow or a plainer print crosses between occasions most easily.
