A gerund is a verb form ending in "-ing" that functions as a noun. Gerunds can be used as subjects, objects of verbs, and objects of prepositions. Certain verbs like "enjoy", "quit", and "appreciate" take gerund objects. The verb "go" is often followed by a gerund phrase like "go shopping". Special expressions like "have fun", "spend time", and "have trouble" can also be followed by gerunds to express how an action is performed.