A smoker is someone who smokes tobacco or sometimes other things like marijuana. They might have a regular habit of smoking.
When we use this word to talk about people who smoke cigarettes, it usually means they've developed a bad habit that's not good for their health. We often hear about efforts to reduce the number of smokers in a city or country because smoking is linked to serious diseases like lung cancer and heart problems. If someone you know is trying to quit, you might call them an ex-smoker once they've stopped completely.
How common is "smoker"?
Word smoker is considered uncommon in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
noun
A person who smokes tobacco habitually.
A smoking car on a train.
(informal, dated) An informal social gathering for men only, at which smoking tobacco is allowed.
(informal, dated, UK, Cambridge University) A social event featuring sketches, songs, etc., whether or not smoking is carried out.
A vent in the deep ocean floor from which a plume of superheated seawater, rich in minerals, erupts.
(slang) An illicit boxing match; see Wikipedia:Battle Royal (boxing).
A device that releases smoke intended to distract bees; a bee smoker.
A person or an apparatus that smokes food.
(slang) A two-stroke engine.
(slang, by extension) Any vehicle with a two-stroke engine, especially a motorcycle, as opposed to a four-stroke motorcycle or stroker.
(baseball, informal) A fastball.
(slang) Synonym of stag film.
A tobacco pipe that produces smoke well when used.