fondant tastes like marzipan or like marshmallows, usually. its “icing” but really its more like a sugar dough that you can tint and then roll out and drape over your cake. you can also use it to make decorations in any shape.
think of it as edible plasticine.
as examples, see these:
http://www.simplycreativecakes.net/documents/fondant.jpg
http://www.giftsandgadgets.ab.ca/images2/Fondant_Class_025.jpg