The aspect of a verb is determined by whether the action is on-going or completed. Although all verbs in the past have already happened, aspect is used to emphasise whether the action was on-going or completed at the time. The four aspects are: simple aspect (also known as the indefinite aspect), perfect aspect (or complete aspect), progressive aspect (or continuing aspect) and perfect progressive aspect.
He took the photos.
(simple aspect - no emphasis of completed or on-going action)
He had taken the photos by the time the owner arrived.
(perfect aspect - action completed)
He was taking the photos when the owner arrived.
(progressive aspect - action on-going)
He had been taking the photos before the owner arrived.
(perfect progressive aspect - action on-going but then finished)
Glossary of grammatical terms