A cylinder is a 3D solid with two equal, parallel circular faces and one curved surface. The key measurements are radius $r$, diameter $d$, and height $h$, with $d=2r$.

Two circle facts come up again and again: area $A=\pi r^2$ and circumference $C=2\pi r$. If the question gives the diameter, halve it before substituting into any cylinder formula.

The curved surface can be unwrapped into a rectangle. Its width is the circumference $2\pi r$ and its height is $h$, which is why surface area involves $2\pi rh$.