When you look at something, light rays are actually reflecting back off that object and into your cornea. This is the clear front of your eye ball which naturally bends the light and allows it to easily pass through the pupil, which your iris will expand and contract depending on how much light there is.
Once through, the light will pass through the eye's crystalline lens. This works in the same way a camera's lens would, shortening and lengthening its width to properly focus the light rays to a point exactly on the eye's retina. At this point the image is actually upside down, but your brain will reinterpret the image for you and allow you to see the world the right way up.
Check out our Human Eye Tool for an in depth look at how we see.