depending on who made the head, but a good axe is forged by using a lower carbon content steel for the main body of the axe- more malleable- and less prone to breaking- then the working edge of high carbon steel is forge welded into the head.
there are a few different processes and ways to do this. but the end result is the same.
but if it is a high production model( like mine is!!) then it is exactly as john says- the head is all one type of high carbon steel, and the lines are just where the working surfaces were heat treated.