They consist of large pictures and very few words, probably only a few hundred at most. These tend to be very simple stories, both fiction and non-fiction. Pictures are often more important than the words. Picture StoryBooks are what most people think of as traditional picture books. Of late, book publishers are buying manuscripts with fewer rather than more words.
Stories are more complicated, and are usually read to children.
Pictures and words have equal importance in these books. Easy Readers are intended to be read by kids, not to kids. Libraries often have books that reproduce appropriate vocabulary lists for school age children. These are in great demand right now. Written for older children to read by themselves, but for children with low reading levels.