Now if we start the first difference is in the name i.e cray and craw, and for the technical part, both are from the crustacean family of shellfish.
It is a freshwater lobster which grows up to maximum length of 10 cm/4 inch and have claws, it is also known as ecrevisse, may take as long as seven years to reach adulthood. Confusingly called as crawfish in U.S. where they are a feature of New Orleans cooking. They have a superb flavor and whatever their color when alive, turns into a glorious deep scarlet color when cooked.Over 300 species of crayfish are found in streams throughout Europe, U.S.,Africa and Australia. Only the tail and claw meat is eaten.
|Crawfish or Crawdad|
It is similar to the lobster except that they have spiny shell,chunkier body with no claws, found on rocky seabed in salty water. Also known as crawdads, spiny lobster, rock lobster and langoustes.
The color of their shell varies according to where they come from and may be dark reddish brown, brown with pale spots, blueish green or pink. All turn pink or red when cooked. Crawfish have dense, very white flesh, similar to that of lobster, but with a milder flavor. Females are better in taste. Crawfish from Atlantic are finest and sweetest of all. Crawfish from warmer water can be a little coarse.
Source of Information: The World Encyclopedia of Cooking Ingredients by Christine Ingram
So before you start on me saying that the above information is wrong, please feel free to refer the above mentioned book and then comment. Thanks