A verb is an action word.
There are different types of verbs, for example linking verbs, action verbs, transitive verbs, intransitive verbs, ect. All of there verbs signify an action or an existence.
"I ran" has the subject I and the verb "ran" because I am doing the action of running.
"She is happy" has the verb "is" because she exists in a state of happiness.
Here is a good site about the types of verbs and some verb rules.