Although similar in looks, with their long thin blades, there are some subtle difference between Filleting and Boning knives that ensure they are perfectly designed for their specific purposes.  A Filleting Knife has a long, flexible blade that is designed to separate meat from skin, whilst a Boning Knife has a thicker, less flexible blade which is designed for detaching meat from bones.

The additional flexibility in the filleting knife makes it ideal for filleting fish, dealing easily with the delicate skin and flesh, as well as the smaller bones.  The thicker, more rigid blade of the Boning Knife gives the additional strength needed for dealing with tougher meats and working around larger bones without damaging the blade.