Deleting an email moves it to Trash, and marking it as spam moves it to Spam. You can bring an email back from either folder, or empty them to delete everything in them for good.
Steps
Desktop
Trash and Spam are under More in the sidebar.
To mark an email as spam, open it and press !.
To rescue an email from Spam, select it, press M (or right-click › Move to) and choose Inbox.
To restore an email from Trash, right-click it and choose Restore to Inbox.
To empty a folder, open Trash or Spam, click Empty Trash or Empty Spam at the top, and confirm.
iPhone, iPad & Android
Trash and Spam are under More in the menu.
To mark an email as spam, touch and hold it and tap Mark as Spam.
To rescue an email from Spam, touch and hold it and tap Not Spam.
To restore an email from Trash, touch and hold it, tap Move to Label and choose Inbox.
To empty a folder, open Trash or Spam, tap Empty Trash or Empty Spam at the top, and confirm.
If you deleted or marked an email as spam by mistake, undo it right away: press Z on desktop, or tap Undo on your phone.
Good to know
Emptying Trash or Spam deletes the emails from your mail account for good. It can't be undone.
Emptying the combined Trash or Spam empties it for every account in your unified inbox. Accounts you've hidden from the unified inbox aren't touched.
Gmail deletes mail in Trash and Spam by itself after 30 days. Outlook and other providers follow their own rules.