Non-Finite Forms of the Verb. The Gerund.

A nonfinite verb is any of several verb forms that are not finite verbs; that is, they cannot serve as the root of an independent clause. Nonfinite verbs found in English typically are infinitives, participles and gerunds. (They sometimes are called verbals—although that term has traditionally applied only to participles and gerunds.) Nonfinite verbs typically are not inflected by grammatical tense; and they present little inflection for other grammatical categories as well. Generally, they also lack a subject dependent. One or more nonfinite verbs may be associated with a finite verb in a finite clause, composing the elements of a verb catena, or verb chain. Because English to a large extent lacks inflectional morphology, the finite and nonfinite forms of a verb may appear the same in a given context. In such a case, the environment surrounding the verb must be examined to determine whether it is finite or nonfinite. A non-finite verb (also known as a verbal) is the term used to describe a verb that is not showing tense. In other words, it a verb form which is not acting like a verb (or, at least, the type of verb you need to form a sentence).
There are three types of non-finite verbs: gerunds, infinitives, and participles. Look at these examples:
I hate camping. (Camping is a non-finite verb. In fact, it is a gerund, i.e., a noun formed from a verb. The giveaway for a gerund is the -ing ending.) I want to go there. (To go is a non-finite verb. It is an infinitive, i.e., the base form of a verb. The giveaway for an infinitive is often, but not always, the to before it.) We ate our roasted marshmallows. (Roasted is a non-finite verb. It is a participle, a type of adjective. There is no real giveaway for a participle, but lots of participles end in -ed and -ing.) The gerund is a non-finite verb form that can function as a noun in Latin and English grammar. The English gerund ends in -ing (as in I enjoy playing basketball). The same verb form also serves as the English present participle (which has an adjectival or adverbial function) and as a pure verbal noun. Thus the -ing form in the English language can function as a noun, verb, adjective or sometimes adverb; in certain sentences the distinction can be arbitrary. The gerund is the form that names the action of the verb (for instance, playing is the action of " to play"). In some cases, a noun ending in -ing sometimes serves as a gerund (as in I like building / I like building things, I like painting / I like painting pictures, and I like writing / I like writing novels), while at other times serving as a non-gerund indicating the product resulting from an action (as in I work in that building, That is a good painting, and Her writing is good). The latter case can often be distinguished by the presence of a determiner before the noun, such as that, a, or her in these examples.