Simple answer: Clean Water, an Aggregate and Portland Cement.

 Aggregates can be any of the following: sand, stone, crushed Coral,foam, glass, shredded paper, peanut shells, shredded rubber,  straw, pretty much any solid organic or inorganic substance.

 There are  several types of Portland Cement, most commonly used is Type I or Type II. Other types of cements are available for specific purposes.

There are many  additives available that can be used to achive different objectives,  such as, water reducers, retarders, excellerators, plastiziers, strengtheners, plastic or carbon fibers or any number of other more sofisticated additives.

Manufacturing of Concrete has become a complex Science due to the increased use of additives and its World Wide use.