See also Article on Fiddleheads
Fiddlehead ferns are a stage in the growth of all ferns - the tightly coiled young shoot. They resemble the head of a violin, hence the name.
Edible fiddleheads are the fiddleheads of the Ostrich Fern. Be careful, because there are many look-alikes (including the Bracken Fern and Royal Fern), which have been shown to cause stomach cancer in a relatively short time (2 to 5 years).