Session is a sequence of one visitor page views. If there was more than a predetermined time between views (session timeout) these views are belong to different sessions. Session timeout is configured in thin settings and defaults to 30 minutes. Session timeout in the current report - 0 minutes.
Page views per session — the number of one visitor page views during one session.
Session duration — the time difference between first and last views of one visitor during the session.
Time spent — the total duration one visitor sessions during the selected period. Attention, if the visitor had several sessions, the time between sessions is not included.
Single visit means that user have visited only one page. In this case the duration of view is not determined.