Why am I seeing the letter S in words that I was taught to spell with a Z?

Okay, maybe I am missing something. Is it Old English? Words that I was always taught to spell with a z are being spelled with an s, in newer books, on t.v., on Fluther…

ex:
realize—realise

civilization—civilisation
this is just a couple, there are more

Did I miss something and it has always been this way or is there just not a wrong or right way? I have always been intelligent but this makes me feel like I have been under a rock