A gerund is one of three types of verbals. A verbal is formed using a verb, but it functions as a different part of speech in the sentence.
Gerunds function as nouns in the sentence. Typically, a gerund is used as a "thing" or an "idea," and gerunds always end in "-ing". They can function as subjects, direct objects, objects of the preposition, and predicate nouns.
Gerunds are not the only types of verbals that can end in "-ing," so make sure that the word you find is functioning as a noun in the sentence.
Gerunds can appear by themselves, or they can be part of a larger gerund phrase.
Examples of gerunds or gerund phrases used as the subject:
Singing is one of my hobbies.
Running is good exercise.
Watching television does not burn many calories.
Examples of gerunds or gerund phrases used as direct objects:
I enjoy cooking with my mom.
Mark likes playing on the team.
The teacher appreciated my trying the math problem.
Examples of gerunds or gerund phrases used as objects of the preposition.
Leslie got into trouble for talking to Sarah.
Have you ever read a book about riding a bike?
Upon finding a piece of glass on the floor, Jan called the teacher.
Examples of gerunds or gerund phrases used as predicate nouns.
A good career for me might be nursing.
My favorite pastime is reading books.
The most fun I had on the trip was swimming in the ocean.