A verb is usually a word that describes an action (such as 'he plays golf', 'she studies economics'). More technically it's that part of speech by which an assertion is made, or which serves to connect a subject with a predicate. This technical definition includes the most frequent verb in the language: the verb 'to be' which can be used to connect a 'subject', such as 'he', 'with a 'predicate', such as 'good at hockey' so the full sentence is 'he is good at hockey'.

There are verbs which take an object ('he kicks the desk'), which are called transitive verbs. Other verbs do not, and are termed intransitive verbs ('I sit, he lives'). Some verbs can be used either transitively or intransitively: 'I sing' is an intransitive usage; 'Paul McCartney sings "God save the Queen"' is a transitive usage. The main verb is the verb on which the structure of the sentence depends, and without which the sentence would not make any sense. In the following sentence the verb 'fell' is the main verb: 'The boy, who had run too quickly, fell'.