Depending on your version of Outlook and the type of message, you'll see different messages in the Infobar.

Outlook 2002: The Infobar tells you if the person is online using Windows Messaging.

Outlook 2002/2003: The Infobar reminds you if you have extra line breaks removed from the messages. When a message has Voting enabled, a note in the Infobar reminds you to vote. It tells you when you replied to a message or forwarded it. If flags and reminders are set, it shows information about the flag and reminder.

Outlook 2003 only: The Infobar tells you if the message was converted to plain text--clicking on it allows you to convert just that message back to HTML. If you aren't automatically downloading Internet content, the Infobar reminds you to click on it to download the missing images and other content.