C is the third letter of the Latin alphabet; it can sound like S or K, and using each sound helps reading.
The soft C sounds like S before the letters E, I, and Y (as in cent, city, cycle). The hard C sounds like K before A, O, U, or at the end of a word (as in cat, cup, clock). There are a few exceptions, and C can join with other letters to make different sounds.
Some words keep the K sound where you might expect S (like soccer or Celt). These words show there are exceptions, and C can make other sounds when it sits with other letters (like ch, ck, or tch).
A common digraph is 'ch,' which usually makes the CH sound in 'chair,' but can sound like K in 'chorus' or SH in 'machine' depending on the word's history. 'ck' often represents K after a short vowel, as in 'duck.'
When the Romans brought C to Britain, it stood for a hard /k/ sound. Before front vowels e and i, C began to sound softer (palatalization), and by the 1200s many Cs before e or i became a soft /s/ sound.
In Roman numerals, C means one hundred.
The lowercase c stands for centi-, meaning one hundredth; for example, 1 centimetre equals 0.01 metres.