What is Quality Score?

According to Google, Quality Score is Calculated from:

• The historical clickthrough rate (CTR) of the keyword and the matched ad on the Google domain; note that CTR on non-Google sites (such as AOL.com) only ever impacts Quality Score on our search partners - not on Google
• Your account history, which is measured by the CTR of all the ads and keywords in your account
• The historical CTR of the display URLs in the ad group
• The quality of your landing page
• The relevance of the keyword to the ads in its ad group
• The relevance of the keyword and the matched ad to the search query
• Your account’s performance in the geographical region where the ad will be shown
• Other relevance factors