Well,
A cylinder is actually a special type of prism. A prism is a 3-D figure made by taking a simple closed plane curve and translating it across space in a certain direction and for a given distance and then connecting the preimage and the image. A cylinder is just a prism made with a circle.
We know from translations that the image is congruent to the preimage. Thus, the bases of a (circular) cylinder are two congruent circles.