Exposure part 1

As I said yesterday a camera is just a lightproof box, simple cameras provide little control over the light entering the box.
They will work in reasonably bright light and everything from about 3-5 feet (90-150 cm) will be in focus. You have to remember that the camera is a precision instrument, it will record what it sees.
Your eyes on the other hand change focus and adjust to different light levels without you noticing. If you wait long enough and have normal eyesight you will be able to see in almost total darkness. The eye can handle an enormous range of light intensity, but even the best film, or digital, camera has a much more limited range.
We can compensate for this by adjusting the exposure in two ways. Increase the aperture, or slow the shutter speed, when it's dark; reduce the aperture, or increase the shutter speed, when it's bright.
Professional photographers have cameras that give them total control. This costs money, but there are ways to achieve very good results without buying a very expensive camera.
If you have a simple digital camera try this experiment, with it set on automatic (A, or P):
On a reasonably bright day go outside and point the camera at your house, or apartment building. Make sure that you include some sky in the picture and have the sun in front of you, but behind the building. If the building is too big, photograph something else. The actual subject doesn't matter, it just has to be reasonably big, not shiny and not too bright.
You will notice that the building is brighter than it looks, and the sky will be almost white.
Try again, but this time point the camera at the sky, so that the subject is below the central auto-focus frame.
The building will now be too dark, but the sky will be correctly exposed.