The stock is the back portion of a long gun. They can also be referred to as shoulder stock or buttstock and may be found on smaller guns like SMGs. A traditional rifle stock provides the structure for the firearm and houses the barrel and action. The purpose of the stock is to provide the shooter with a structure for the shooter to brace against the body which aids in aim and stability. The stock also helps absorb some recoil which counters muzzle rise in real firearms. On modern tactical firearms, stocks can be adjusted for length on the fly by design. Collapsing stocks are useful for different sized players or when you are wearing a lot of bulky gear which would normally make it difficult to shoulder the rifle.
As a general rule for smaller stature players or younger ones, guns with a collapsing stock rather than a full or side folding stock is ideal. Side folding stocks keep the overall size of the gun down but may or may not have additional adjustments to improve the ergonomics of the stock once the stock is unfolded and deployed.
In airsoft, stocks also serve an additional function by often times housing the battery and wiring harness for the airsoft gun. Depending on the style of stock you have, you may be limited in selection what kind of batteries you can use. Often times guns with full stocks will have the largest compartment so you can fit a wide variety of batteries. SMGs and compact guns might come with very minimal stocks that do not house any type of battery and is purely single purpose. You will find these guns will typically be front wired or have small compartments in the back that are practically hidden (like on the Classic Army X9 for example).
A common term you might hear when we talk about airsoft stocks is "crane stocks". This is just the nomenclature that has stuck over the years in airsoft that is a generic term for a stock capable of housing an airsoft battery. In reality, the crane stock was just a specific stock found on the M4 rifles used by operators early on and now have become a fairly common buttstock. The crane designation denotes the origin of the stock, which is the Crane division of the Navy.
Another common term you might hear your fellow airsofters say is "buffer tube". The buffer tube is a component on an AR-15/M4 style rifle which houses a large spring and a buffer. This component is vital to the cycling of an AR/M4 rifle. The collapsing stock is mounted onto the outside of the buffer tube. In airsoft the term has carried over though there is no vital function that the tube performs in the operation of the gun.
In this section you will find replacement stocks or stock related items. Certain guns may have available adapters for collapsing stocks where they would normally have come with full stocks or side folding stocks.