The Wikipedia article – http://en.wikipedia.org/wiki/C – says only, “The Romance languages and English have a common feature inherited from Vulgar Latin where C takes on either a “hard” or “soft” value depending on the following vowel.” But why? In classical Latin, “c” always was pronounced “k” – wasn’t it?
And come to think of it, why does the Latin alphabet even have a letter “k”? Why did they need it?
Even weirder – what good is “q”?
K is easier than C. They got K from the Greek kappa, which has the same shape and sound. Since it comes in the same place in the alphabet, presumably they got C from the Greek gamma, which has a somewhat similar shape but a different sound. So wouldn’t the question be, why did the Romans need C when they already had K?
I don’t know about the actual history, but our hard C and G sounds are actually very similar-- the former is an unvoiced velar stop and the latter a voiced velar stop.
K also does the kkk sound. S also does the sss sound. But only C does the ch sound.
I think that the OP is correct that at one point, Latin “c” was always pronounced [k]. What I assume happened is that at some point, there was a conditioned sound change in which the [k] sound became a [s] sound in certain environments, namely, before front vowels (I think). Front vowels in Latin included what is represented as “i” and “e”. So we get:
city
cinder
cite (yeah, I know I should have one)
cent
cellar
etc.
with the [s] sound, and all the other environments:
clamor
crisis
came
come
cure
etc.
with the [k] sound.
I’m sure it’s a lot more complicated than that (and actually, it could be that [k] first became “ch” and then [s] given that in Italian, the combinations “ci” and “ce” are always pronounced with a “ch” sound), but it’s a start.
Alright, I think that’s enough armchair speculation.
Originally in Latin, “K” sounded like /k/, and “C” sounded like /g/ (which is why in inscriptions you see abbreviations like “Cn.” for “Gnaeus”. Eventually, for some reason, “C” became used for both /k/ and /g/, and so the letter “G” was invented to disambiguate the two. “Q” was originally a Semitic letter pronounced in the back of the throat, but by the time it got to Latin via Greek, it sounded like an ordinary /k/. Latin probably kept it around representing the sound /kw/ to disambiguate pairs like “cui” and “qui”.
Before front vowels, “C” in Latin began to be palatalized (pronounced with a /y/ offglide) in all Latin dialects except for those on Sardinia. This wound up giving various reflexes in different Romance languages: Italian, Sicilian, Romanian, Dalmatian, etc. pronounced it like English /ch/; French originally pronounced it like /ts/ and then changed it to /s/ (whence the English sound); Spanish pronounced it like /ts/, changed that to /th/, and in some areas, changed it further to /s/.