User Action: Aggregates User Actions by their action
type (click, key press or load action). Page load actions are grouped
further by the requested URI.
Visually Complete: A page is visually complete when all the visual components have been
rendered. It is the time when a user considers that the page is ready to
interact with. It doesn't consider, for example, any Ajax request that
could be going on behind the page. Visually complete can be captured for
page load and for Ajax actions.
Review the below links for more understanding.
To add to this, the user action response time is simply when the onload event completes during the page load. This page has some additional information on the timings:
Visually complete is a more complicated method of trying to identify when the page appeared to be completely loaded for the end user. More goes into this so Babar's document will help with a better understanding of it. Note that you may come across perceived render time which is a similar measure that attempts to track when the page appeared to be loaded but visually complete is a more accurate version based off or more recent additions to modern browsers.