It works as a noun, and it comes from a verb. We call it a gerund. If I give an example in Polish, you might understand better: śpiewać --> śpiewanie Beware! Not all [verb]ing forms are gerunds. If it is part of a continuous verb form, or if it works as an adjective, or if it starts a short phrase which gives extra information about something, then it is NOT a gerund, but a present participle. We make them exactly the same way as gerunds, but the structure of the sentence tells us whether we have a gerund or a participle. You need to look at the complete sentence.

This is a gerund form where we are using a word that could be a verb as a noun. Also like swimming, skiing, running

Smoking is used as a noun in this case to refer to the actual act of smoking. Not all gerunds are used as nouns, though. For instance, "I am reading now" uses the gerund of the verb "to read" i.e. "reading" to refer to an act that takes place right now. Other examples: Reading is a hobby of mine (--> used as a noun) I love swimming (--> used as a noun) versus I was reading 3 hours non-stop yesterday (--> used as a verb) I am taking this opportunity (--> used as a verb)