This seemingly simple question has a lot of different answers, since not all eyes work like humans’ do! The mantis shrimp, for instance, has SIXTEEN kinds of photoreceptors for color, most that are invisible to us (we’re stuck working off only three: red, green, and blue). They can even see UV, visible and polarized light, can move each eye independently and can see how light moves in a circular motion!
Here’s how human eyes work (in a nutshell):
All around the eyeball, ocular muscles help move your gaze. The clear cornea bends incoming light so that it goes through the pupil or black center point of your colored iris. The iris around the pupil opens or closes like the shutter on a camera to allow more or less light through to the pupil. After that, light passes to the lens, which focuses the beams before the go through the jelly-like vitreous or aqueous humor that is inside your eyeball and keeps it round. Ciliary muscles change the shape of the lens to help us see near or far. Your retina at the back of your eye, made up of light-sensitive cells called rods and cones, receives the light and transforms it into electrical signals that can then be transferred to the brain via the optic nerve.
So, we see with both our eyes AND our brain. An amazing thing is that the visual cortex of our brain is also active in blind people, even though images aren’t delivered to them by the eyes. So, even in humans, there’s lots of different ways to “see”!