It is most commonly used as a gerund, which is a verb acting as a noun. For example:
* "Skiing is my favorite winter activity." (Here, "skiing" is the subject of the sentence.)
It can also be used as a present participle, which describes an action in progress. For example:
* "She is skiing down the mountain." (Here, "skiing" describes what she is doing.)
However, it's important to note that "skiing" is not the infinitive form of the verb. The infinitive is "to ski."