When the letter "c" sounds like an "s", it is said to be a soft sound, such as in the words "cedar" or "cider" or "cypher". When the letter "c" sounds like a "k" it is said to be a hard sound, such as in the words "cat" or "coffee" or "cup".
Generally, (although there are always exceptions) a "c" will be soft when it is followed by the letters e, i or y.
The same rule applies to the letter g.