A session differs from a visit in a way, that it is defined as a group of interactions – not just pageviews – performed by the visitor of the website during a timeframe. Each time a visitor performs a desired action, it counts as a session. These interactions can be mouse movements, clicks, scrolling motions, form fills and more. Sessions have an expiration time. After 30 minutes of inactivity, or after the revisit happens from a different traffic source, a new session starts.
Example: Your potential visitor finds an article online. In that article there is an anchor text or Call-To-Action that directs traffic to your website. After clicking on it, you’ll gain a visitor. The visitor browses more of your content, then stops being active for 30 minutes without closing your website. After 30 minutes – and the first session – expired, the visitor becomes active again (e.g. browses more content, subscribes to newsletter). This will count as a second session but number of visits is still one.