A session is tracked from site open to site close, or when someone ‘backgrounds’ the sites for 15 seconds or more (maybe they opened another tab). Sessions are really useful when combined with other metrics to analyse true engagement rates.
Sessions also have metrics for length of session and intervals between multiples. This can start to build a picture of how engaged a user is, based on behavioural patterns.
When a page is requested by a browser, a Pageview is logged. This tracks individual pages, whereas a session encompases multiples. This is useful to show which pieces of content are popular.