Content Editor
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Lets you author HTML content through a simple browser based HTML editor or a source
code text window.
Form
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Allows you to collect user input using traditional html form controls and pass the
data entered to another Web Part.
List
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View Displays a WSS list through Collaborative Application Markup Language (CAML).
For every list created on a site a List View Web Part is automatically available
to be added to a page.
Each standard list template (Links, Announcements, contacts, events, tasks, issues,
document & image library) has a predefined default view for the List View web part.
The List View Web part can also render any custom view defined by a user for a list.
So, if you 100 lists in your site, you will have 100 List View Web Parts.
Members
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Displays the presence information for all users who have participated in or have
been added to the current site along with their IM status.
Page Viewer
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Effectively an iFrame that can display any web page, file folder or physical file.
Web Page Title Bar
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A Web part included on all web part pages that stores the page title and description
(this part isn't displayed in the web part gallery)
XML
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Allows you to embed or link to some XML data and embed or link to the XSL to render
the XML (can be used to display RSS news feed etc)
XSLT Data View
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The most powerful web part which can only be configured by FrontPage 2003. It allows
you to get data from a SharePoint List, XML file, Database or Web Service and render
it using XSL.