In order to understand several of your questions, you need to understand the concept of exposure in photography.
Film Speed
For a given speed of film (or sensor sensitivity with digitals), it requires a certain amount of light to reach the film in order for the image to be properly exposed. I like to use a plumbing analogies in this area. Assume it takes "a gallon of light" to properly expose an image with 200 speed film. When you double the speed, it requires 1/2 the amount of light to expose the image. When you 1/2 the speed it requires double the light to expose an image. So if you needed a "gallon of light" with 200 speed, you'd only need a "1/2 gallon" if you switched to 400 speed or "2 gallons" of light if you dropped down to 100 speed.
"So why use lower speeds?" The answer is mainly image quality. As you use higher speed films, you get "grain" in the images. When you jack up the sensitivity on you digital camera, you get "noise" (like "snow" on TVs).
Aperture
Each lens has an aperture that controls the amount of light that passes through the lens. It works the same way as the iris in your eye. In the plumbing world, this would be like the size of a pipe. The larger the pipe, the more water can pass through it. The unit of measure for camera apertures is the "f-stop". Like the film speed, when you go "up" an "f-stop" the lens will allow double the amount of light through the lens, and when you go "down" a stop it cuts the amount of light in 1/2. The "f-scale" goes like this: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 32, 64. A lens set at f5.6 allows twice the amount of light to pass through as one set at f8, and half as much as one set at f4.
Each lens has a "maximum aperture" based on lens design, the size of the elements, etc. A f1 lens is a theorhetical "perfect" lens. The "maximum aperture" of a lens is usually printed on the front of a lens. If the lens say "50mm, f1:1.8" it means that "wide open" the lens can do no better than f1.8. On a zoom lens it may read "70-210mm, f1:4-5.6". This means that the maximum aperture changes as the lens is zoomed in and out. At 70mm it's f4 and at 210mm it's f5.6.
Shutter Speed
This is the time that the shutter is open to allow light to hit the film (or sensor). This is the time that the faucet on your sink is open. It's usually measured in fractions of a second. If your shutter speed reads "250" on your camera, that's 1/250th of a second.
Tying it all together
So you decide that you need a "gallon of water" to to make a photo. You can do that with two variables: aperture and shutter speed. There are multiple solutions to your problem. If you have narrow pipes, you can leave your faucet open longer... or if you have fatter pipes you can turn your faucet off sooner. Both ways you end up with a "gallon of light". An exposure of 1/60th @ f4 is the same as 1/30th @ f5.6 (in the second example you've narrowed the aperture by half, but compensated by doubling the length of time the shutter stays open).
There's one more piece to the plumbing analogy... water pressure. In photography, that's the amount of light in area where you are shooting. A brightly lit room is like high water pressure, and a dim room is like the house at the end of a water main. The lower the "light pressure" the wider the pipes you're going to need or the longer you'll need to keep that faucet open to get that gallon of light.
The thing that decides how wide to make the pipe and how long to keep the tap open is the camera meter. Using the "light pressure" it sees coming through a "wide open" lens, it computes the desired aperture and shutter speed needed to get the light it needs to the film to make a correct exposure.
What film speed to use
Basically, you need a film that's fast enough to make sure that the meter can use a shutter speed that's fast enough to not make blurry images. The maximum aperture of your lens can often be a factor in film selection. If I have a lens with a maximum aperture of f4, I can get away with using film that's half the speed of when I use a lens with a maximum aperture of f5.6. Remember: an f4 lens can pass twice the light of an f5.6 lens when the lens "iris" is wide-open.
This is one of the reasons that people ought to take the maximum aperture of a camera's lens into account when making a purchase. The greater the maximum aperture of a lens, the more flexible is it. This will come into play in lower lighting situations.
Back to the "speed" question... It boils down to shutter speed. The old rule-of-thumb in 35mm photography was that you shouldn't use a shutter speed that is slower than the reciprocal of your lens length. If you were using a 200mm lens, then you'd start to see "hand shake" in the image at below 1/200th of a second. If you dipped below that, you needed to open up the lens more or get faster film.
Generally, 200 speed is a good "general" speed to use. Night time with flash, 400 or 800. You can get great fireworks photos with 100 speed (the fireworks themselves are very bright). 1600 speed is pretty much special use stuff in my book... for low light with no flash. I pretty much use it in dim churches when the pastor doesn't want flash used during a wedding.
