I was going to just spell check antichrist-'s guide... but that got boring so I just kinda rewrote it. It leaves some things out that antichrist- had in his, but whatev.


ISO:
ISO (or ASA which is used with film) is the sensors(or films) sensitivity to light. The higher the ISO, the more sensitive the sensor will be to light. When you use higher ISO, the sensor doesn't need as much light to create the image - but higher ISO creates more noise. You should try to use the lowest possible ISO to maintain picture quality. However, you are not always able to use a low ISO and still take a picture using a desired shutter speed and aperture.

Aperture:

Aperture is often represented with a "F/#" like F/2.8, F/8, F/22. Basically, Aperture is how wide or narrow the hole in your lens can be. The wider the hole in the lens, the more light the camera will let through to the sensor. Likewise, the narrower the hole is, the less light can get to the sensor. The aperture also affects 'DoF' of Depth-of-Field. DoF is represents how much of a picture is in focus. A wide open aperture like F/1.4 will let in a lot of light, but it will also create a very small depth of field. This wide aperture will leave a very small part of the picture in focus. The contrast is that a very narrow aperture like F/22 will allow little light through, but it will allow you to get a bigger area of the picture in focus.
The numbering system for aperture is the opposite of what you would expect. For example:

A picture taken at F/1.4 is a very wide open aperture and it will let in a lot of light but will create a very shallow depth of field.
A picture taken with an aperture of F/22 will be a very small, narrow hole that will let in very little light and will create a very large DoF.
All lenses will have a minimum and maximum aperture. A good low-light lens will have a small minimum aperture (like F/1.4 or F/2.8). The lower the 'F/#' - the FASTER a lens is considered and the better it is for low-light photography.


-Prime lenses are lenses that do not have any zoom. An advantage to this is that they are generally cheaper than zoom lenses and are very fast lenses. An example of a Prime lens is the Nikon 50mm F/1.8 lens. The lens costs roughly $100 new and is a very fast lens that allows a lot of light in. Prime lenses are good for things like portraits and low-light sports photography. The Nikon 300mm F/2.8 is a very common lens that is used by professional sports photographers. Although you cannot adjust the zoom on this lens (it is forever set at 300mm), it is a very fast, expensive, and large lens.

-When looking at zooming telephoto lenses, the prices will generally depend on how low the aperture can go on the lens. For example: the Nikon 55-200mm F/3.5-5.6 is a lens that will only go down to F/3.5 when shooting at 55mm and F/5.6 when shooting at 200mm. This lens costs roughly $200. The Nikon 70-200mm F/2.8 covers almost the same zoom range but will allow you to shoot at F/2.8 no matter if you are shooting at 70mm or 200mm. This lens is roughly $2,000.
A general rule with lenses is that you do not want to shoot at whatever the minimum aperture is on the lens if you want the sharpest pictures. For example, if you have the 70-200mm F/2.8 lens, you would want to shoot at around F/3.5 or higher for clearer results. All lenses have a "sweet spot" and you will have to find it depending on the lens you pick.



Shutter Speed:
Shutter speed is how long the camera's sensor will stay open and receive light. Shutter speed is generally seen written as a fraction of a second. When you see '1/500' - this is implying that the sensor will stay open for 1/500th of a second. This is fairly fast. A shutter speed that is written with quotes after the number is showing seconds. A shutter speed of : 20" means that the sensor will stay open of 20 seconds and allow a lot of light in. The BULB setting on most cameras means that the sensor will stay open as long as you are holding down the button. General rules with shutter speed are:
a.) If you are taking pictures of moving subjects, you will need a faster shutter speed. If you are taking pictures of people running or playing sports, you will want to keep your shutter speed at around 1/320th of a second or higher. (These are all suggestions and these numbers will change depending on how fast the subjects are moving)
b.) If you are going to use a very slow shutter speed like 2" (Two seconds) - you will need a tripod or a steady surface to make sure that your camera does not move in those 2 seconds. Any vibration or movement during those 2 seconds will show up in the picture as blurring.
c.) Do not try to hand hold your camera if you are shooting under 1/25th of a second. This is generally the point where natural vibration from holding the camera will show up in the picture and it will turn out blurry. Even with Vibration Reduction and Image Stabilization, any lower than 1/25th of a second is generally time for a tripod.
d.) Try to not use VERY long shutter speeds like 10-20 minutes as the picture will come out very noisy and it can cause harm to your camera's sensor. Most camera sensors will show pink/purple blotches if the sensor starts to overheat.


How do all of these things work together?
Shutter speed, aperture and ISO are all required to make an exposure (take a picture). If you imagine the shutter speed as a curtain that opens up for a certain amount of time (either for a very little amount of time like 1/1000th of a second, or for a long time like 30 seconds). The light from your subject passes through the curtains and then is limited depending on if you have a wide open aperture like F/1.4 or a very narrow aperture like F/22. If you have your shutter speed as low as you can go for a certain situation and the aperture is as wide as it can go - but you are still not getting a 'bright' enough picture, you need to adjust ISO. ISO allows you to make the sensor more sensitive to the light that is hitting it. All of the light that passes through the shutter and aperture is then recorded onto the sensor. This is the basics that you'd need to understand while changing ISO, Shutter speed, and aperture.



A few scenarios to help demonstrate these settings are:
1.) If you are shooting a football game and are using only the stadium lights, your settings might look something like :

Shutter Speed : 1/250th (So that you avoid blurring while allowing light to get to the sensor)
Aperture : F/2.8 (You will generally have to set this as wide open as possible to get as much light as you can in)
ISO : 800 (This is saying that your shutter is as low as it can go and your aperture is as low as it can go. The only thing you can add is the added sensitivity to light of the sensor. Try avoiding the use of ISO if possible, but do not lower shutter speed to make up for it. Noise can be removed in post-processing, motion blur cannot be removed)


2.) If you are shooting a city light scene at night, your settings might look like:
Shutter Speed : 30 seconds
Aperture : F16
ISO : 100/200 (200 is the lowest on some cameras)

This is saying that your shutter will let in light for 30 seconds and the aperture will be very narrow. This will give you a bigger depth of field. ISO is not needed since motion blur will not be a huge issue with city lights and you can compensate with shutter speed.


To sum it up, you basically are trying to find the right balance of these 3 main settings in order to get the picture that you want under the conditions that you are given. You can theoretically compensate by increasing or decreasing any of the settings. In example #1, you could use something like a shutter speed of 1/320th of a second, an aperture of F/4 and an ISO of 1600. Since I made the shutter stay open for a shorter period of time and made the aperture hole smaller, I have to make the sensor more sensitive to light to compensate for it.

I didn't really read this over and left some stuff out, so I'm sure that there are some mistakes - feel free to correct them.

And again, credit goes to antichrist- for the original version that can be found here : http://forums.d2jsp.org/index.php?showtopic=35470476&f=265