A cylinder is two parallel circles joined by a curved tube. Soup cans, drinking glasses, batteries, drums — cylinders show up wherever you want strength with simple parts.
Volume
V = π r² h
Base area (πr²) × height
Surface area
A = 2πr² + 2πr h
Two circle ends + the curved side.
Unroll the curved side and you get a flat rectangle — width 2πr (the circumference), height h. That's why the side area is 2πr·h.