Mark a card as completed and find it in the Completed work tab, you can press “c” to see your completed task day by day.
If you are not done with a card but don't want it on the desk you can either snooze it see 5) How to snooze Cards or “remove it from the desk".
Cards unmarked as done can be found in Uncategorized, if they do not have a tag, or in the respective tags, if you have added one to the card.
“Uncategorized” and “tags” are findable in the left nav (either if collapsed or open)
