By default, Desktop Alerts are turned on. The information displayed in a Desktop Alert varies depending on the item that you receive in your Inbox. Displays the name of the sender, the subject, and the first two lines of the message. A Desktop Alert doesn’t display the contents of an encrypted or digitally signed message. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.