Silverlight is a browser plug-in that supports multimedia content. It also is a slimmed-down, cross-platform version of Microsofts Windows Presentation Foundation (WPF) programming model. Each successive iteration of Silverlight includes more and more WPF functionality (and vice versa).
Windows Phone 7 Series, which is Microsoft's revamped smartphone operating system, and Silverlight, the company's rich Internet application platform, might seem geared primarily too glitzy, for-fun applications. But Microsoft observers see an enterprise business application case for these two budding technologies. (Silverlight is being paired with the XNA gaming app dev platform to serve as the application development platform for Windows Phone 7 Series.)
Silverlight 4 adds support for data binding, enterprise networking and printing, and lots of other features that are likely to make the platform more appealing to folks writing not just single-function, lightweight Web apps, but enterprise apps, as well.
Silverlight also is Microsofts primary development environment for Windows Phone 7 devices, but the current Silverlight mobile dev platform is a hybrid of Silverlight 3 and Silverlight 4, not pure Silverlight 4. Microsoft officials have said that Silverlight wont be running (as an Internet Explorer plug-in, at least) on the first Windows Phone 7 devices that ship by this holiday season.
One important caveat for developers, as acknowledged on the Silverlight download site:
Microsoft Visual Studio 2010 can be installed side-by-side with Microsoft Visual Studio 2008 SP1. For Silverlight 4 development, you will need the released version of Microsoft Visual Studio 2010. Please read the known issue on installing Microsoft Visual Studio 2010 if you already have the Silverlight 3 SDK installed.