Buffer tube: First, be aware that the AR's buffer spring, buffer, and bolt carrier ride back and forth in what is commonly called either a buffer tube, receiver extension, or receiver extension tube. I personally call it a buffer tube, but I've seen all three terms used interchangeably.
Buttstock assembly: As used online, 'buttstock assembly' often consists of only the moulded buttstock, the rear buttstock plate, the lower buttstock screw, and a sling catch. What's missing is the buffer tube, the upper buttstock screw, and the recoil buffer itself - which is frequently fairly expensive in its own right. When purchasing a buttstock, make sure you know what's in the package - if it looks like a killer deal, it's probably missing parts.
The Recoil Spring and Buffer
While there are a handful of configurations where the recoil spring and buffer are unnecessary, in most cases they are essential to the proper operation of an AR-15. The CZ V-22 upper and the Bohica Arms single-shot upper do not require the use of a recoil spring and buffer. There are probably more uppers which don't require a buffer. However, the vast majority of uppers are regular semi-auto uppers designed to function with a recoil spring and buffer. Omitting these parts would result in, at best, the bolt carrier being wedged into the buffer tube and being difficult to extract. At worst, it could break the gas port of the bolt carrier off or damage the rear of the upper.
Choosing a Buttstock
Why you would want a telescoping stock
You can press a lever on the telescoping stock and bring it from short to long and back again in no time. A compact rifle is frequently easier to transport, and it certainly holds a sophisticated, modern appeal. Most of the rifles depicted on the evening news in Iraq are M4s with telescoping stocks. Most telescoping stocks have six positions, although some earlier models and current cheap models have only four positions. These multiple positions allow the rifle's length to be adjusted for the shooter's comfort, and to trade off between manoeuvrability and accuracy. Telescoping stocks generally range from $50-$300, and all have SOME degree of clockwise / counter-clockwise play with the shoulder mounted part of the stock. The higher-priced stocks, however, tend to have such a small amount of play that you wouldn't notice it unless you were specifically looking for it. Of note to Californians is that if you choose a telecoping stock, you must fix your magazine in place unless your rifle falls into another exception area (i.e. it's either a rimfire rifle or is not semi auto). Incidentally, one reason to be wary of cheap collapsible stocks is that there are stories of some cheaper stocks which have such a weak spring holding them in place, that they actually collapse and shorten themselves under the recoil of a fired shot.
Why you would want a fixed stock
Fixed stocks are, by definition, non-adjustable. They're firm against your shoulder, generally lacking 'play' as is commonly found with telescoping stocks. Fixed stocks have a nostalgic appeal (due to the classic M-16 design) and are mandatory for some Service Rifle competitions. Most A2 style butt stocks have a 'trap door' in the rear, which can accommodate a cleaning kit or a lead weight. The weight may be used to counter-balance a heavy barrel or shift the center of gravity rearward for an almost bullpup-like feel - but in either configuration, the added weight reduces felt recoil considerably. Other types of fixed stocks incorporate a storage slot for a spare magazine, not unlike the storage compartmet of the Kel-Tec SU-16. One consideration for Californians is that fixed stocks are not an 'assault weapon' feature, which meaning that if you want to have detachable magazines on your semi-automatic, centerfire AR, you have to use a fixed stock of some sort.
The military A2 buttstock is made of nylon, while the A1 buttstock was made of fiberglass.
Buttstocks are typically depicted as being all black, but they arrive as a light gray. The owner is expected to coat the stock with CLP if a sleek, black finish is desired.