Shock absorbers, also known simply as "shocks" (which is what we will call them for the rest of this article), are the suspension components which stop your car or truck from bouncing up and down after hitting a bump. They are a major contributor to your ride comfort and safety.
"Struts" are an integral part of your suspension, usually combined with a spring and other pieces to support the wheel and tire. A strut contains a shock within it, so a strut performs a similar function to a shock. A shock is a separate, "stand-alone" item in your suspension, while a strut contains a shock AND performs additional functions.
Big Answer #1: All Struts Are Shocks, But Not All Shocks Are Struts.
Next, you may ask yourself "what does my ride have, shocks or struts?" While we cannot list every vehicle within this article, we can give you this general information: the majority of late-model cars, and even light-duty trucks, have struts in the front, and shocks in the rear. There are exceptions of course. Some vehicles have struts both front and rear. A few vehicles have shocks, not struts, at all four wheels. There is always ONE shock OR strut at each wheel. Whichever design your vehicle has, you cannot replace "struts" with "shocks", or vice versa, when performing a service replacement.
Big Answer #2: Vehicles Have Either A Strut Or A Shock At Each Wheel; You Cannot Have Both At The Same Wheel; And When Replacing, You Always Replace Shocks With Shocks, And Struts With Struts.
One reason this has been confusing to the public is that service professionals sometimes use the terms "shocks" and "struts" interchangeably, which is incorrect. You may have been told "your car needs new struts" even if your car doesn't have struts. You could also be told that you need new shocks when in fact your car has four struts.
So you've determined that your vehicle has struts, and you're further confused to discover that the replacement component might be just a "strut", OR a "strut cartridge", OR a "complete strut assembly"! What in heck is the difference?
Let's take the most common replacement item first, and that is just the "strut". This strut consists of a lower housing, with the shock contained within it. When performing the replacement, the repair person reinstalls the original spring onto the new strut. WARNING! This repair involves special spring-compressor tools, and is potentially dangerous. This is the most common strut replacement, as the vehicle gets a new strut housing, new spring seat, as well as a new internally-contained shock.
If you see that a "strut cartridge" is available for your ride, this means that the technician can disassemble your original strut, remove the internally contained shock (the strut cartridge, similar in concept to the ink cartridge inside a ball-point pen), install the new cartridge, and reassemble the strut. Again, let us warn you that special tools are needed! The replacement of just the cartridge can save you money in parts, with the trade-off that more labor is involved.
Finally, a recent option which has become popular is the "complete strut assembly" replacement. This consists of the strut housing, self-contained shock (or cartridge), coil spring, and upper mount. One big advantage is that a special spring-compressor tool is not needed. You also have peace of mind knowing that all the strut-related components are replaced at the same time. The part will cost more, but should take less labor time to install.
Big Answer #3: A "Strut" Replacement Could Consist Of Just "The Strut", Or, The "Strut Cartridge", Or, The "Complete Strut Assembly", Depending On What's Available For Your Vehicle, And How Much You're Willing To Spend.
The replacement of shocks is much more straightforward. Shocks can usually be removed and installed without disturbing the springs, so special tools are not needed. It's best to lift the vehicle for good access. You may see options such as "gas charged" or "mono-tube", which are examples of higher performance (read: firmer ride) shocks compared to original equipment (OE) quality parts. Parts as well as labor costs are typically lower for shocks compared to struts.
Big Answer #4: Shock Replacement Can Usually Be Done With Common Tools, Although Raising The Vehicle Will Either Be Necessary Or Greatly Add To The Job's Convenience. Performance-Style Shocks Can Be A Nice Upgrade Over OE Quality Shocks.
Whether your vehicle has shocks or struts, if you plan on keeping your ride for a while, buy the best quality items you can. Always replace shocks or struts in pairs! If you have over 100,000 miles on the clock and are still on the factory parts, consider changing all 4. You'll be happily amazed at the ride and handling improvement!