Axles are often overlooked until they break.  After that, they become one of the most scrutinized parts within any axle housing.  An axle by definition is a bar or rod that transfers torque from the differential to the wheel.  Here are common terms that describe axle construction, differences in strength and their uses. 

 
Materials:
Carbon Steels are forged materials used by OE Manufactures and can vary in quality and durability.  The 10xx family of steel is the most common with 1040 and 1050 making up the bulk of factory and replacement axles.  Many aftermarket axles are a 1541 carbon steel.  Upgrading the quality and strength over the OEM axles.            
 
Alloy Steels are basically Carbon Steels with certain chemicals added to improve the performance in specific applications.  The 43 in 4340 refers to a family of high strength low alloy steels, sometimes called Chromoly.  A 4340 steel would have somewhere between 0.38% to 0.43% Carbon.  Making 40 its designation within this family of steels.  The term Chromoly comes from use of Chromium and Molybdenum as strengthening agents.  Other popular Alloy Steels are 4130, 4140 and 4145.
 
Spline Type:
A Rolled Spline axle has the splined "rolled" into shape.  Under pressure the axle end surface has groves forced into it, making the valleys or splines, and the displaced material is compressed upward into the "teeth".  The compressed steel teeth have a tight grain or structure providing good strength at the base.  Heat-Treating follows to provide a durable surface.
 
Cut Spline axles are what most people associate with custom axle production.   The splines are machined or cut into the axle end surface.  The majority axle blanks used for this type of process have a deep heat treat to provide an excellent surface for the splines to be cut from.  The deeper heat treatment also provides great strength.  This allows for custom cut axles without sacrificing durability.
 
Axle Type:
Full Floater axles are axles that drive a hub at the wheel.  Usually found on ¾-ton and larger trucks.  Theses axles have drive flanges on one end of the shaft and axle splines on the other.  They do not support the weight of the vehicle and the axle bearings do not ride directly on the axle surface. 
 
Semi Floater axles are axles that help support the weight of the vehicle and directly drives the wheel & tire.  Found on most passenger cars and light trucks, this axle design has a wheel bearing directly contacting the axle surface.  
 
C/Clip and Bolt-In Axles:
Semi Floater axles can be retained or held in the axle housing one of two ways.  A C/Clip is exactly what it sounds like.  A "C" shaped clip that attaches to a grove on the end of the axle and keeps the axle from coming out of the housing.  The 8.5" GM axle is a common C/Clip style axle.  Bolt-In axles have an axle bearing that is pressed onto the axle shaft.  A retaining plate then holds the wheel bearing into the housing end.  The Ford 9" is a common Bolt-In style axle.