The idea behind shape notes is that the parts of a vocal work can be learned more quickly and easily if the music is printed in shapes that match up with the solfege syllables with which the notes of the musical scale are sung.
Whether or not shape notes actually facilitate learning music is disputed. Most modern participants in shape note traditions would probably argue that they do. On the other hand, newcomers to shape note singing who can already read music may feel that the shapes do not help, though the task of learning to use them might perhaps be enjoyed as a novel musical challenge.
lots more + pics in the wiki page :
http://en.wikipedia.org/wiki/Shape_note