When the te-form of a verb [that describes activities that last for some time] is followed by the helping verb いる, we have a sentence describing an action in progress.... You can also use a ~ています sentence to describe what a person does by occupation or by habit....
[Another group of verbs describes] changes from one state to another. If you get married, or 結婚する, for example, your status changes from being single to being married. With these verbs, ている indicates a past occurrence of a change which has retained its significance until the present moment. In other words, ている describes the result of a change.