I’ve never been master of all the different styles, I pick up a lot of tidbits from my lolita friend and at anime conventions I work at.
But from what I’ve gathered fairy kei is focused on pastel color scheme from head to toe. Lots of cute accessories. Especially a hell of a lot more hair accessories then the general lolita style uses. There are usually a lot of cute characters highlighted in the style (Strawberry Shortcake, Disney characters, Care Bears, Hello Kitty, also cute food, etc). Usually have tutu-esque skirts and tights. Loose shirts are pretty common as well. Also, cotton candy colored curly wigs are really common.
Compared to the more general lolita style. Which has the cute frilly dresses, coupled with socks, shoes, and appropriately matching bag. Limited accessories that are more carefully chosen for the outfit. There’s a lot of kinds of lolita styles though.
One of the main differences in the styles is that lolita outfits always seem to be very carefully constructed for the desired look the person wants. Fairy Kei can be constructed, but it comes off as more of a mix and match and cover in trinkets kind of look.