What is the difference between a surah and an ayah?

Surah: a chapter

The word surah (plural suwar) is usually translated as "chapter". Each surah has a name, often taken from a word or story in it: al-Baqarah (The Cow), Yusuf (Joseph), al-Kahf (The Cave). Surahs vary hugely in size, from 286 verses in al-Baqarah to three in al-Kawthar. Every surah except at-Tawbah begins with "Bismillah ir-Rahman ir-Rahim".

In the Quran, Allah challenges anyone who doubts it to "produce a surah like it" (2:23). Even the shortest surah counts, which shows how much weight a single chapter carries.

Ayah: a verse, and a sign

An ayah (plural ayat) is one verse. In a printed mushaf, each ayah ends with a small circle holding its number. But the word ayah literally means "sign" or "miracle". The Quran uses the same word for the verses and for signs in nature, like the night and day, so each verse is seen as a sign pointing to Allah:

How to read a Quran reference

• 2:255 means surah 2 (al-Baqarah), ayah 255. That is Ayat al-Kursi. See What is Ayat al-Kursi and when do I recite it?
• 1:1–7 means the whole of surah 1, al-Fatiha, which has seven ayahs. See Why is Surah al-Fatiha in every prayer?
• Some apps write "Surah 2, Verse 255" or "al-Baqarah 255". It's the same thing.

Other divisions you'll hear about

A juz is one of 30 equal parts, used for reading plans, especially in Ramadan. A hizb is half a juz. A ruku' is a short passage marked in the margin of many South Asian copies. These divisions are just for convenience; surahs and ayahs are the Quran's own structure. Some ayahs also carry a sajdah mark, where you prostrate. See What do I do when I reach a sajdah verse in the Quran?

Every letter counts, too. The Prophet ﷺ said that whoever reads a letter from the Book of Allah gets a good deed, multiplied by ten (Jami' at-Tirmidhi 2910).