One concept that's sometimes a bit unclear to new Gmail users is that of "Archiving" messages. I'm not refering to the hype and false assertions of pundits who claim things like "Gmail keeps everything you ever send and receive!" and "Gmail archives your messages forever!" No, I'm refering to the "Archive" button in your Inbox view. Just what is it for, and what does it do?
Plain and simple, clicking Gmail's "Archive" button simply means that the message gets moved out of your Inbox--nothing more, nothing less. It's a convenient and slick way to manage your Inbox.
But just what happens to the message? If you applied a Label to the message and then click "Archive" button, the message will no longer be visible in the Inbox. It will be visible in the applied Label view as well as the "All Mail" view. If no Label was applied, it is removed from the Inbox and is only viewable in the "All Mail" view. In any case, all messages, regardless of how thery are classified, can be viewed in the "All Mail" view. Any Labels that have been applied are shown in the listing. If the message hasn't been "Archived" yet, it will see "Inbox" Label in the "All Mail" view.
Basically, when you "receive" a message, it automatically gets assigned an "Inbox" Label. It's a Label just like any other Label. The only difference is that the Label is just typically hidden while viewing the Inbox. The "Inbox" Label is, however, visible in other views. To test this, take any email that's in your Inbox and apply a Label, but do not Archive it. Go to the view of the Label you applied, and look at the message listing. You should see both the Label you applied as well as an "inbox" Label. It's also visible in the "All Mail" view. When you Archive a message, it simply strips off the "inbox" Label so that it doesn't show up in the Inbox view.
A little tip is that if you are in any other view other than the inbox view, you can always select a message, click on the "More Actions" dropdown, and click on "Move to Inbox" to move it back!