A camera is basically a light-proof box. It has a lens system to focus light onto the film at the back of the camera. Let's pretend we are outside on a beautiful summer day trying to take a picture of the family dog. This means that there is only a single set of lenses for both viewing and photographing an image. You point the camera at him.
Light passes through the camera lens and hits a mirror.
The mirror is on an angle so it reflects light up onto a horizontal glass focusing screen where the dog's image is formed. Light is then reflected through a pentaprism, a piece of glass with five sides. The pentaprism corrects the upside-down and reversed image. When we see the dog's image in the viewfinder eyepiece, it's the right way up. Now let's say that the dog looks really cute panting on the grass, and you decide to snap a picture.