How to Guides
Migrate local folders from Thunderbird to Gmail (PDF)
Gmail Keyboard shortcuts
A note about messages, labels and the sent label.
All messages are saved in All Mail, excluding Trash and Spam.
Everything else (Inbox, Sent Mail, Starred, custom labels, etc) are just views into a sub-set of the messages in All Mail. And since there's only one copy of any message (Gmail doesn't have folders), if you delete it from any label, you're deleting the only copy, so it's gone from all of them.
If you delete a label you remove that "view", but the messages are still in All Mail.