Hi Chris,
From what I'm lead to believe, the 'bar' in the idle bar plug was to stop the element becoming 'saturated' at idle and thus flaming out. I'm sure someone will correct me if I'm wrong there.

The hot/cold thing is where it gets interesting though.

The different heat designations are 'supposed' to be an indicator of what type of fuel they're suitable for (specifically the methanol content of the fuel). Unfortunately, the 'cold/hot' designations aren't always the same between brands. A much better way of identifying different plugs would be by element thickness. This way, they can't be confused. Thicker element for low methanol content fuels (more platinum available for the catalytic reaction), thinner element for higher methanol content fuels.

The thicker the element, the more platinum surface area is available for the chemical reaction that keeps it glowing. Lower methanol content fuels require proportionally higher platinum in order for the reaction to be sustained. However, if you look at the element in, say, an Enya#3, it's actually quite thick, even though it's called a 'hot' plug

As for the long or short plugs, well, the difference there should be obvious No idea of the virtues of either though......I assume some engines prefer having the element further down in the head (or visa versa), thus requiring a 'long' (or 'short') plug.