XBAP defintion from MSDN:
"XAML browser applications (XBAPs) are intended to combine the best
features of Web applications and rich-client applications. As with a
Web application, XAML browser applications (XBAPs) can be deployed from
a server to a system with a single click. The application then runs in
the browser without any interruptions beyond the time needed to
download the application. However, unlike the relatively limited
graphics and media capabilities of HTML-based applications, XAML
browser applications (XBAPs) have access to the rich features of the
Windows Presentation Foundation (WPF) runtime."
Silverlight definition from MSDN:
"Silverlight combines multiple technologies
into a single development platform that enables you to select the right
tools and the right programming language for your needs. Silverlight
offers you the following features:
-
WPF
and XAML. Silverlight includes Windows Presentation Foundation (WPF)
technology, which greatly extends the elements in the browser for
creating UI. WPF lets you create immersive graphics, animation, media,
and other rich client features, extending browser-based UI beyond what
is available with HTML alone. Extensible Application Markup Language
(XAML) provides a declarative markup syntax for creating WPF elements.
-
Extensions
to JavaScript. Silverlight provides extensions to the universal browser
scripting language that provide powerful control over the browser UI,
including the ability to work with WPF elements.
-
Cross-browser,
cross-platform support. Silverlight runs the same on all popular
browsers (on any platform). You can design and develop your application
without having to worry about which browser or platform your users have.
-
Integration
with existing applications. Silverlight integrates seamlessly with your
existing JavaScript and ASP.NET AJAX code to complement functionality
you have already created.
-
Access
to the .NET Framework programming model and to associated tools. You
can create Silverlight-based applications using dynamic languages such
as Managed JScript and IronPython as well as languages such as C# and
Visual Basic. You can use development tools such as Visual Studio to
create Silverlight-based applications.
-
LINQ.
Silverlight includes language-integrated query (LINQ), which enables
you to program data access using intuitive native syntax and strongly
typed objects in .NET Framework languages.
-
If
you already use ASP.NET, you can integrate Silverlight with the server
and client capabilities of ASP.NET that you are familiar with. You can
create server-based resources in ASP.NET and use the AJAX capabilities
of ASP.NET to interact with server-based resources without interrupting
the user."
So, you might ask what the big difference is?
Silverlight or WPF/E is meant to be cross OS and cross Browser compatible.
The ADOGuy has more on this
debate.