In English, gerunds are -ing verbs that are used as nouns.
Gerunds are really cool, but people mistake what they are all the time. Since we're total grammar nerds, we wanted to give you reasons why you should be super excited about them. Because you really should be. Seriously.
There are literally zero exceptions. All gerunds in English look like this: running, skipping, hopping, writing, looking, singing, guffawing, gnawing, Shmooping, snooping, souping, etc. Take any base form of a verb, add -ing, and it can be a gerund. (Well, if you take into account spelling rules.)
So why are gerunds so cool? Well, chew on this first: nouns are things and ideas that exist in the universe, and words are their labels. A cat is a thing, and every language has a label for it: cat, gato, chat, katze, neko, billi, köttur (we could go on forever). But really, everything in the universe is a thing. If you see something and want to talk about it, you can...by making into into a noun (or "nominalizing").
We bet someone saw their friend crying, and they made the word "sadness." By adding -ness, they turned an adjective ("sad") into a noun. They probably said, "What is that sadness coming out of your eyes?"
But the act of crying (a verb) is probably something they wanted to talk about, too. That's when you'd use a gerund.
Ugh. So cool.
Keep in mind that pretty much every language has some way of nominalizing verbs. It just so happens that English uses the present participle (-ing form). Check out our examples and common mistakes to get a better idea of what a gerund is.