A camera is an optical instrument that captures a visual image.
At a basic level, cameras consist of sealed boxes (the camera body), with a small hole (the aperture) that allows light through to capture an image on a light-sensitive surface (usually photographic film or a digital sensor).
Cameras have various mechanisms to control how the light falls onto the light-sensitive surface.
Lenses focus the light entering the camera.
The aperture can be narrowed or widened.
A shutter mechanism determines the amount of time the photosensitive surface is exposed to light.