The simple explanation of concrete is:

Concrete is a composite stone type material made of aggregate (sand and gravel) bonded together  by adding Portland Cement.  The various sizes of aggregate is what gives concrete much of its strength as the cement encapsulates the aggregate to become a hard and reasonably stable building material. It’s important to remember that there can be no such thing as concrete without adding water to the mix to create hydration.

The American Concrete Institute defines concrete as:

mixture of hydraulic cement, aggregates, and water, with or without admixtures, fibers, or other cementitious materials.

 And

a composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregate, usually a combination of fine aggregate and coarse aggregate; in portland-cement concrete, the binder is a mixture of portland cement and water, with or without admixtures.”