What's the difference between Login Time and Last Activity Date in User System Usage?

In the User System Usage subject area, the Login Time is the actual time the user signed in and started a session through the web or CX Sales Mobile UIs.

The Last Activity Date is the last date there was any activity for that user, including workflow notification emails, and BI reports that could be running with the user's ID. Because the Last Activity Date covers any activity related to the user, it can be later than the last time the users was actually signed in.