What is a verb?

The definition of a verb is a word that forms the central action of the sentence. For example, in the following sentence:

The quick brown fox jumps over the lazy dog.

"Jumps" is the verb. Every verb has a subject; what it applies to -- in this case the subject is "the quick brown fox". Verbs can also have adverbs and objects -- which modify the verb, or specify what the verb acts on. In this case that is "the lazy dog."