Archiving and deleting cards
Archive cards by clicking "Archive Card" in the Actions menu on a card's back. Or you can also select "Archive" from the card menu (click the pencil icon that appears on hover). You can also archive with the "c" keyboard shortcut when hovering your mouse over a card.
Archive cards you no longer need on your board.
Click "Menu" then "Archived Items" from the board sidebar, find the card and click "Send to Board".
The search bar will bring up a list of archived cards that match your terms. You can use the same special operators as the search bar—see Searching for Cards (All Boards) for a full list.
You can delete a card from the back of the card. Click the card to open it, then choose "Share and more" from the bottom right. You'll see a delete link in the bottom right of the menu.
Deleting cards is permanent.
You can also delete from the archive. Click "Archived Items" in the board's menu, find the card and select "Delete".