The trash folder is cleared of messages when you quit Kmail if this option is selected.
All messages queued in the outbox folder will be sent whenever you check for new mail.
With this option, a confirmation is automatically send to the sender of a message if he requested that. If you disable this option, no confirmation will be sent.
This will compact all folders every time you exit Kmail. Compacting folders means that the messages you removed from a folder will really be removed.
To empty a folder means that its messages get transferred to the trash folder (and only get deleted if they are already in the trash folder). So although you normally won't lose any mail, you can still activate a confirmation dialog that will ask you if you really want to emtpy the folder.
If you don't like the Composer you can use any editor. Note that the Composer window will still open and the external editor will open as soon as you type just one character in the body of the message. If you're done, save the text and exit the editor. The text will now appear in the Composer window.
This let's you configure actions for the arrival of new mail. Note that automatically fetching mail can be configured in your account settings under Incoming Mail.