You're a smoker and you can't do without your daily dose of nicotine? Be reassured, it won't be an issue in Iran. Even though smoking is prohibited in Islam, there are no regulations against cigarettes in Iran.
Everywhere in the country, it's rather common for people to smoke cigarette in the streets, and even in cafes and restaurants. If you are travelilng to a more conservative city, simply pay attention to people around you, or ask if you have any doubt about smoking in a specific place.
However, it's not as common for women to smoke in public in Iran. Yet many of them do, especially in Tehran where this is totally acceptable, but it might be surprising for locals in smaller cities.
You can buy cigarets, at a really cheap price, in most groceries, supermarkets, and at the small kiosks in the streets. Most occidental brands can be found, while "Bahman" is a typical Iranian brand.
Whether you are a smoker or not, you might appreciate smoking "Ghelyoun" in Iran. Ghelyoun is the name in Farsi for water-pipes, also called "Hookah". It contains flavoured tobacco, at different tastes, from mulberry to watermelon, and is usually served with tea and rock candy. It's truly part of the Iranian culture to share Ghelyoun between friends or family, at some restaurants and even picknicks.
It's usually available in traditional restaurants. If you go to a specific Ghelyoun place, pay attention to the fact that it may be for men only. Otherwise, at restaurants, it's perfectly fine for women to smoke.
Also, note that whereas it can be called "shisha" in some countries, this word has bad connotations in Iran, and it's preferable not to use it - it means metamfetamine (crystal) in Persian!
In short, there is no specific restriction on smoking in Iran. It’s authorized in most cafes and restaurants, where can also be served the traditional Ghelyoun.