To add just a little bit to Vivian's spot-on answer, and to tease out something she kind of says and say it more explicitly:
I often see gerund defined as "a verb which functions as a noun". Specifically, it is, as Vivian mentions, "a verb ending in -ing which functions as a noun." This differentiates it from an infinitive ("a verb preceded by the word to which functions as a noun"; ex.: "To load the truck is easy because of its design") and from a participle ("a verb ending in -ing or -ed which functions as an adjective"; ex.: "The running man passed the truck").
Part of what causes this little cluster of grammatical fun is that our suffix -ing is collapsed from two different endings in Old English/Anglo-Saxon: -ing/-ung which was used to make gerunds and verb forms indicating that some action had been completed or habitual, and -ende which was used to form present participles. By about six or seven hundred years ago, the two had both been collapsed into our suffix -ing. In Latin, for example, all of these forms have very different and distinct endings.
Hope that helps just a little bit more!