The above weapon is an US-made M4A1. Notice the steel telescoping sliding stock at the rear, that allows the user to adjust the length of the weapon as needed.
The next weapon we see is an AK-47 with a folding aluminium stock. Notice that the stock attaches to the back of the rifle with a hinge that allows it to fold sideways. Also note that the stock is a skeletal frame. This is to reduce the overall weight of the weapon. Unlike the previous weapon, this one doesn't allow the shooter to adjust the length of the stock to the user's needs though.
It must be noted though that a metal stock does not always imply that the weapon is lighter. A skeletal steel stock is usually heavier than its wooden equivalent. Most metal stocks are also usually coated with a rubber or plastic material on the outside.
Metal stocks can be easily produced in large quantities via mass production techniques. Due to advances in machining technology, they can be produced to very accurate dimensional tolerances. They also have the advantage of being very cheap in price, compared to stocks made of other materials. Metal stocks generally provide superior rigidity than other types of stocks, but offer less in the way of dampening and absorbing shock. Some metal stocks have a shock absorber built in to handle the shock. Metal stocks will expand and contract with temperature variations, which affect accuracy. On the other hand, humidity does not have any effect on them.