A verb is a part of speech. More specifically, a 'doing word' -- a word which describes an action (I go to London) or a state of being (I like ice cream, I think, therefore I am). Depending on the language, a verb generally varies in form according to many factors, possibly including its tense, aspect, mood (or "mode"), and voice, as well as the person, gender, and number of its subject.
There are several major types of verbs. These include intransitive; transitive; ditransitive; and ergative, all of which types are marked in some of the world's languages. Caucasian languages not only mark verbs for ergativity, but also have ergative-absolutive noun case systems. Several languages spoken in Papua New Guinea mark verbs for transitivity, and some also mark verbs for ditransitivity.