School me on primes.
I've heard on a few forums that primes are on their way out since modern zooms (high end ones) are so good now.
Not a chance. The market for primes is shrinking, true, because consumers are finding zooms to be "good enough", but they will
never disappear. I know somebody who owns a 5D; he bought the 85mm f/1.2L recently (worth over $AU3000, thank you very much). Three thousand Australian dollars for a single lens, with a single focal length. Not something I'd do - I don't need an 85mm lens, and if I did, I'd probably go for the f/1.8 instead - but still.
Or look at the professional sporting matches. Most of the lenses you'll see there will be 300mm, 400mm, 500mm, and 600mm primes.
You will
never find a zoom lens faster than f/2.8, for example, yet there are times when having a lens at f/2.0 or faster is exceedingly useful. You'll also probably never see a zoom lens that goes longer than 400mm; it would be too heavy to use effectively.
What sort of shots are primes good for?
Wrong question. A given prime will have a particular use. For example, an 85mm lens on a full frame body (like the 5D or 1Ds series) is a fantastic length for portrait work and similar. A 24mm might be used for architecture. A 135mm might be used for sporting events with the local school team, and a 1200mm is probably going to be used for bragging rights.
You would pick the prime you want to buy based upon what you want to do with it - you wouldn't just grab any old lens and then ask "what is it good for?" Especially when the fast, long primes can go for five figure sums (400mm f/2.8L: $AU10,000 plus ... and that's on the grey market. Bought through Canon Australia, RRP is over $AU13,000. The Canon 1200mm f/5.6L went for over a hundred thousand US, and was a special order.)
Could you potentially use a prime as your every day every situation lens, especially if you don't mind using your feet to zoom?
Yup, there's no reason why you couldn't do that with a relatively wide prime (say, 50mm or wider on a 1.6 crop body). Longer lenses would make it harder to get good shots, although again, it depends on what you're trying to shoot.
What focal length is roughly the same as what the human eye sees?
Depends on the camera. What you're talking about is called a "standard" lens. On a 35mm film camera, 50mm is considered standard. On a 1.5 crop body (any Nikon) or 1.6 crop body, like the 20D, 30D, or 400D, around 30mm would be standard. A medium or large format would have a significantly longer lens classed as standard - a 4x5 large format camera, for instance, would use a 150mm (or so) lens if you wanted a standard field of view. There's a bit of leeway in this - eg, in 35mm format, anywhere between about 40mm and 58mm is considered standard. The rule to derive this, by the way, is to take the length of the diagonal of the image sensor (be it film or digital). That length is the focal length of a standard lens for that particular body.
A shorter focal length than a standard lens is called "wide angle" - it takes in more than the human eye would take in. Longer is called "telephoto", with a much narrower field of view than the human eye - so you can zoom in on birds, or the baseball pitcher, or the bowler in cricket, and so forth.
