Concrete is the combination of 3 materials
Sand is the fine aggregate. Gravel or crushed stone is the coarse aggregate in most mixes.
Portland cement is a fine powder produced heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding small amounts of other materials. The cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together.
Water is needed to chemically react with the cement (hydration) and too provide workability with the concrete. The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the w/c ratio, the stronger the concrete. (higher strength, less permeability)