Oh Camerahs!

Before light reaches film, it must pass through an opening called an "aperture". The aperture is like a pupil. You can control the aperture by setting the "Aperture Opening", also known as an f-stop. —Photonhead

The smaller the F-stop number (or f/value), the larger the lens opening (aperture). —Wikipedia

But aperture also affects an important photographic element called "depth of field" (short form "DOF"). You may ask, what is hell is this "Depth of Field" ? Depth of field is just technical term used to describe the 'zone' of sharpness' between nearest and furthest of a subject in focus (to be more exact, distance of sharp focus in front and behind, subject on which the lens is focused). —MIR

The literal interpretation of the f/N notation for f-number N is as an arithmetic expression for the effective aperture diameter (entrance pupil diameter), which is equal to the focal length divided by the f-number: D = f / N. The notation is commonly read aloud as "eff" followed by the number: f/8, for example, is usually pronounced "eff eight".

The pupil diameter is proportional to the diameter of the aperture stop of the system. In a camera, this is typically the diaphragm aperture, which can be adjusted to vary the size of the pupil, and hence the amount of light that reaches the film or image sensor. The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture. —Wikipedia

My favorite analogy for exposure is filling a bucket of water. A bucket is of fixed size and needs a certain amount of water to fill it, just like film, which is of a set film speed and needs a certain amount of light to capture an image. To fill your bucket, you can pour a small stream of water for a long time or a fast stream of water for a short time. Either way, you end up with the same amount of water. In photography, the size of the stream of the water is analogous to the f/stop, the length of time you pour is analogous to the shutter speed, and the size of the bucket is analogous to the film speed. Broadly speaking, from the bucket's point of view, it doesn't matter which combination of stream size and length of time you choose as long as the right amount of water ends up coming in. Film is the same; within limits, it is indifferent to the combination of time and amount of light as long as the right amount of light eventually arrives. —US Coles

In traditional (film) photography ISO (or ASA) was the indication of how sensitive a film was to light. It was measured in numbers (you've probably seen them on films - 100, 200, 400, 800 etc). The lower the number the lower the sensitivity of the film and the finer the grain in the shots you're taking.

In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography - the lower the number the less sensitive your camera is to light and the finer the grain. Higher ISO settings are generally used in darker situations to get faster shutter speeds (for example an indoor sports event when you want to freeze the action in lower light) - however the cost is noisier shots. —Digital Photography Schools

When you do override your camera and choose a specific ISO you'll notice that it impacts the aperture and shutter speed needed for a well exposed shot. For example - if you bumped your ISO up from 100 to 400 you'll notice that you can shoot at higher shutter speeds and/or smaller apertures. —Digital Photography School

An SLR differs from a point-and-shoot camera in several important ways. The viewfinder on an SLR looks through the same lens that the camera uses to expose the image sensor. This means that your viewfinder shows you a much more accurate representation of the final image, including the effects of any filters you might have placed on your lens.

Most SLRs also use interchangeable lenses. That lets you attach the lens that's exactly right for your shooting needs and upgrade lenses to improve image quality.

Because of their larger size, most digital SLRs are equipped with image sensors that are much larger than those in point-and-shoot cameras. A larger image sensor usually produces better image quality than a smaller sensor because the individual pixels on the sensor can be larger. Larger pixels produce images with less noise, so SLRs tend to yield better results than point-and-shoots when working in low-light.

SLRs also tend to have higher-end features, such as more rugged bodies, faster burst rates, higher ISO settings, and in some situations, better image quality.

Creatively, the main difference between the two types is that it's possible to shoot shallower depth of field with an SLR. —Creative Pro

A friend once said to me that Canons are the best cameras available designed by engineers, and that Nikons are the best cameras one can buy designed by photographers. There may well be some truth to this aphorism.

Not to put too fine a point on it, but after being away from Nikon for the better part of a decade and having been immersed in the Canon gestalt since then, I was frankly surprised at the extent to which current Nikon cameras offer feature and function advantages over Canon. While some of the ones mentioned above may not be relevant to any one photographer's needs, there are bound to be several which can be real productivity aids, if not just downright fun to have and use. —Luminous Landscape