[rmccue]

÷ is the symbol for division since it symbolises a fraction, with the two dots taking the place of the numbers. I'd say this is why, as some people have noted, : can also be used for division in some contexts: it represents a ratio, which is again just a fraction.

[sesqu]

The way I was taught it, though, is that ratios are rather different from fractions. You might say 2:1 of tea and honey, but ⅔ tea and ⅓ honey. However, when describing only one quantity, some people will say 1:2 of honey while others might say 1:3 of honey.