An alloy is a material that is a metal combined with other materials
An alloy is a material that contains a metal, plus other materials. The materials are added in small amounts, but the small addition is enough to change the properties in useful ways.
For example, one very common alloy is steel. Steel is mostly iron with a little carbon. Another common alloy is what we call "aluminum" - the material used in soda cans - which is actually an aluminum alloy. Aluminum is an element on the periodic table of the elements, but the "aluminum" in a soda can also contains very small amounts of manganese and magnesium, and possibly other things, too. Aluminum alloys are used in many things. For example, "alloy" wheels are made from aluminum alloy.
Bronze is an alloy of copper and tin. The discovery of this alloy was so important to civilization that the "Bronze Age" is named after it. Copper by itself is soft, but people discovered that by adding some tin, turning the material into bronze, they created a good material for tools and weapons.
The main ingredient of an alloy is the base metal; the added ingredients are alloying ingredients
The main ingredient of an alloy is called the base metal. In the case of steel, the base metal is iron. For aluminum alloy, commonly called simply "aluminum" in everyday usage, aluminum is the base metal (duh!). The base metal can also be called the primary metal.
The other materials added to the base metal are called "alloying agents". The key alloying agent in steel is carbon. There may be additional alloying elements, too, depending on what type of steel is being made. These alloying elements, when added to the base metal, create a new material that is largely made from the base metal, but has different properties. The effect of an alloying agent can be to add strength to the material, or hardness, or resistance to corrosion, or other improvements. Aluminum in its pure form is lightweight but soft and weak. The fact that it is lightweight makes it good for aircraft, but alloying agents are needed to make it stronger. The aluminum alloy used in aircraft frames is called duralumin.
An alloy improves on the properties of the base metal
We use alloys because they have properties that are useful. The properties of an alloy can be quite different from the base metal that they are mostly made from. For example, iron is soft and brittle, but steel is stiff and incredibly strong. That's why we make skyscrapers out of steel and not iron. Copper and bronze are quite different as well. Copper is very soft and ductile, while bronze is harder and stronger.
Common alloys are used to build jets, skyscrapers, soda cans, bicycles, jewelry and much more.