Gmail has always been pretty good at filtering junk mail into the “spam” folder. But today, in addition to spam, people get a lot of mail that isn’t outright junk but isn’t very important–bologna, or “bacn.” So we’ve evolved Gmail’s filter to address this problem and extended it to not only classify outright spam, but also to help users separate this “bologna” from the important stuff. In a way, Priority Inbox is like your personal assistant, helping you focus on the messages that matter without requiring you to set up complex rules.”
The Priority Inbox segregates Gmail into three different categories: “Important and Unread”, “Starred”, and “Everything Else”. Gmail automatically filters incoming e-mail into either “Important and Unread” or “Everything Else”, while the middle category is populated by those messages that have been flagged for future reference.