Focal length is the distance between the film and the optical center of the lens when the lens is focused to infinity (specified in mm).
The focal length determines the angle of view, short focal lengths giving a wider AOV than longer focal length lenses.

The aperture is the opening of a lens that is controlled by the diaphragm and determines the amount of light that is let through to expose the film. These can be compared to the iris and the pupil of the eye. The iris [diaphragm] controls the size of the pupil [aperture]. In a dark room the pupil [aperture] needs to be wide open to allow lots of light through so you can register an image; the opposite applies in sunlight when it has to close down small to limit light.