If you speak English — and you probably do if you're reading this — an orthography or writing system of "A, B, C, D..." makes sense. The Serbian language uses a different orthography: "A B C Č Ć D Dž Đ..."
From Greek orthographia, meaning "correct writing," an orthography is a way to represent words by having set ways to spell and write them. Orthography helps you identify words so you can figure out their meanings. Different languages usually have different systems of orthography, and even subsets of language, like text-messaging lingo, have a sort of orthography.