Basically:

      In an SLR camera, when you look through the viewfinder, you will see the exact image that will be projected onto the film. This is different than point-and-shoot since you can see what the film will "see" and change any aspect you want, from lens focus to shutter speed. Professionals tend to use SLR for this reason because they can control every feature of an exposure.

     Technically:

     Between the shutter and the lens there is a mirror that reflects the image onto a prism, which rotates and reflects the image through the viewfinder, thereby showing you exactly what the lens is pointing at. When you press down the shutter-button, the mirror flips away and the shutter opens, revealing film to light and taking a picture.

Each aspect of this process can be changed to your preference. By adjusting the lens, you can change what part of an image will be in focus. By adjusting the shutter size, or aperture, you can change how big of a opening light can get through when you take a picture. By adjusting shutter speed you can control how long light is allowed to reach the light-sensitive film.

To get all of things right and achieve the 'perfect' picture is challenging and takes practice. Anyone who has patience and enthusiasm can learn how to use a manual camera to their advantage and create a photograph exactly the way they want it. With automatic cameras, this can be impossible since you control very few of the camera's mechanisms. With digital, it is unnecessary to know any of the mechanisms because you can see the picture immediately after you take it and make changes without using up film.