If you're positive you won't need a message again, use the Delete button to send it to Trash.* 

Deleting unimportant mail is a great way to free up some of your storage, but with Gmail's free storage, you can probably keep those messages, too! If it's possible that you'll need a message or conversation in the future, we recommend archiving.

Archiving mail moves messages out of your inbox and into your "All Mail" label for safekeeping-- you won't be bothered with extra messages cluttering your inbox, but you'll still be able to find a message if you need it six years from now!

*Deleted messages and conversations are permanently removed from Gmail 30 days after you send them to Trash.