Are you done working on a card?
Mark it as completed and find it in the left navigation panel where completed cards are
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 removed 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)