How is the status ‘Opened’ tracked?

In order for an email to be indicated as being "Opened" it must be an HTML email that was opened, images must be enabled, and the recipient is connected to the internet.

If an email isn't marked as being "bounced" then it was delivered.  Of the delivered emails only a portion will be tracked as being "opened".  The remainder either didn't open the email, had images disabled, etc.