A prism is a solid with bases that are polygons and the sides are flat surfaces. (See Definition of a prism). Strictly speaking a cylinder is not a prism, however it is extremely similar. If you imagine a prism with regular polygons for bases, as you increase the number of sides, the solid gets to look just like a cylinder. So we can say that a cylinder is a prism with an infinite number of faces.
In the applet above, keep clicking on 'more' to see this effect. This is similar to the way a regular polygon turns into a circle when the number of sides gets infinitely large. See Area of a circle derivation.
A cylinder is also similar to a prism in that