The basic unit of 'letting in light' in photograph is colloquially referred to as a 'stop'. Actually, it refers specifically to the aperture of the lens - if you recall, the wider the aperture, the more light - and we refer to 'stopping down' a lens to narrow the aperture. So let's start there.
Aperture is measured in what are called 'f-stops', so you'll often see something written as f/8. It's actually the focal length of the lens divided by the diameter, so for example, a 200mm lens with an aperture of 50mm is f/4. Note (as an aside) that making long lenses with low f-numbers is therefore hard and expensive, as you require a lot of high-quality glass - a 400mm f/2.8 lens requires you to produce an optically perfect bit of glass 14cm across! It's why, for example, you pay a LOT more for a Canon 400mm f/2.8 lens than a 400mm f/5.6 (ten times more!)
The other thing to note is f-stop is a function of diameter, and (obviously enough) light transmission is a function of area. A f/4 lens actually lets in four times as much light as an f/8.
If you look at a lens marked with f-stops, the conventional scale goes like this (it's slightly rounded, but effectively you're multiplying by the square root of 2 each time):
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32Each step to the right halves the amount of light let in (go do the maths if you want!), and obviously each step to the left doubles it. This doubling/halving of light is what's referred to as a stop.
Shutter speed is measured in fractions of a second (obviously enough). Halving or doubling the shutter speed, obviously enough, halves or doubles the amount of light let in. Again, there's a conventional scale with some slight rounding:
1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, etcand as before, one step either way is a stop.
'Film' sensitivity is measured in a number of ways, but the one that (after a while) the camera folks seem to have standardised on is know as the ISO rating. As I said before, in a film camera (except for some special films) its a property of the film itself, but in a digital camera you can control it from shot to shot. In a nutshell, the higher the ISO rating, the 'faster' a film, i.e. the more sensitive to light. A doubling of ISO rating counts as one stop, and 'normal' film (the stuff you used to buy for your Instamatic) is ISO 100. [There is also a version of the ISO scale where ISO 100 = ISO 21°, and a doubling increases the ISO degree by 3, but most cameras don't seem to use that these days.]
So, then. It turns out that, for example, on a fairly bright day, you can set your SLR camera up for ISO 100, 1/250th of a second and f/8 and you'll get a decent shot. But you can do exactly the same thing at (say) ISO 400, 1/250th, f/16 or ISO 100, 1/500, f/5.6... all three parameters interact, and you can fiddle with them to get the same exposure.
How do the point-and-shoot Instamatics do it? They're basically set to about the equivalent of that - ISO 100, 1/250th, f/8 - and they rely on most photos being taken outside in reasonable conditions, and the fact that film is actually fairly forgiving and they can fix it a bit when they develop the film!
Equally, in an artificially lit wargames room I find myself generally shooting at about ISO 1600, 1/30th, f/2.8 and making compromises, without a tripod.
Next time up, I'll produce some practical examples of those compromises, to do with depth of field, camera shake, graininess etc, and what you can do to overcome them.
For now, have fun, and I hope this was educational!