What is the difference between Clicks and Sessions? Why there are discrepancies on Google Analytics on these dimensions? Normally you will have much more clicks than sessions, but there is a simple reason for this: these metrics are different.
Click is the most basic metric. When a user clicks on an ad, that click is always measured.
A Session is an event on Google Analytics. Imagine that the user clicks twice on your ad. You will have two clicks however only one session if it was within an interval of 30 minutes. Google Analytics sessions are measured over a period of 30 minutes.
How a session expires?
After 30 minutes of inactivity
If a user arrives via one campaign, leaves, and then comes back via a different campaign.