A mixture of metals is called an alloy. Brass is an alloy made of copper and zinc. There is no fixed amount of these two metals. Varying the proportions gives brass with slightly different colours and properties.
Brass is used in many decorative objects because it has a gold appearance. It is also resistant to corrosion and so has been used for door locks, keys, mechanical watch parts, screws and other fixings. It is also used for many musical instruments such as trumpets.
Brass melts at a lower temperature than iron and becomes very runny, making it easy to cast. Brass has no iron and so it cannot be affected by magnets. This has made it useful in some areas. Having no iron means that if struck it will not produce sparks. This makes it useful to use around explosives.
If aluminium is added, it becomes even stronger and weatherproof. The same is true if tin is added. This is why brass is still used on ships and boats - it does not corrode in seawater.
Adding lead makes it a bit less hard and easier to shape with a cutting machine. Lead is rarely used nowadays because of its polluting effect on the environment.