Portland cement is the fine gray powder that, mixed with water, binds sand and gravel into concrete. It is made by heating limestone and clay to roughly 2,700 °F, then grinding the resulting clinker with a little gypsum. Cement is an ingredient, not the finished material: concrete is cement + water + aggregate, and cement is only about 10–15% of a typical mix. A “cement sidewalk” is really a concrete sidewalk.
In the US, portland cement is specified under ASTM C150. The common types are Type I (general use), Type II (moderate sulfate resistance), Type III (high early strength, for cold weather or fast form stripping) and Type V (high sulfate resistance). Most bags at the store are labeled Type I/II, meeting both. A full bag weighs 94 lbs — one cubic foot of loose cement. Bagged “concrete mix,” such as an 80 lb Quikrete bag, already contains cement, sand and gravel; you only add water.
Concrete gains strength through hydration, a chemical reaction, not by drying out. Two rules follow: use the least water that still lets you place and finish the concrete (a lower water-cement ratio means stronger concrete that cracks less), and keep it moist while it cures. Concrete typically reaches about 70% of its design strength in 7 days and its rated strength — 3,000 or 4,000 psi for most residential work — at 28 days.