The fourth option in the Delete Browsing History window deals with Form data. Anytime you enter information into a form on a webpage, Internet Explorer stores some of that data. For example, you may have noticed when filling out your name in a form that after typing the first letter or two your entire name becomes populated in the field. This is because IE has stored your name from entry in a previous form. Although this can be very convenient, it can also become an obvious privacy issue.
The fifth option deals with Passwords. When entering a password on a webpage for something such as your email login, Internet Explorer will usually ask if you would like for the password to be remembered. If you choose for the password to be remembered, it will be stored by the browser and then prepopulated the next time you visit that webpage.
The sixth option, unique to Internet Explorer 8, deals with InPrivate Blocking data. This data is stored as a result of the InPrivate Blocking feature, which informs you about and gives you the ability to block webpage content that is configured to observe your personal browsing history. An example of this would be code that could tell a site owner about other sites that you have recently visited.