Microsoft Silverlight was a discontinued application framework for creating rich internet applications (RIAs). Think of it as a less powerful, browser-based cousin to WPF (Windows Presentation Foundation). It allowed developers to create interactive web applications with animated graphics, video playback, and other features using a programming model similar to WPF, primarily based on XAML (Extensible Application Markup Language) for the UI and C# or VB.NET for the backend logic.

Essentially, it provided a way to deliver visually appealing and interactive content within a web browser, much like Flash did (though Silverlight used different technology). However, unlike Flash, it was developed by Microsoft and had stronger integration with Windows systems.

Key features (past tense as it[SQ]s discontinued) :

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Cross-browser compatibility (partially):

While aiming for cross-browser compatibility, it wasn[SQ]t as universally supported as HTML5 eventually became.
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Rich media capabilities:

Excellent for streaming video and incorporating interactive elements.
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.NET framework integration:

Developers familiar with the .NET framework found it relatively easy to work with.
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XAML-based UI:

Allowed for clean separation of presentation and logic.

Why it[SQ]s discontinued:

Silverlight was ultimately superseded by HTML5, CSS3, and JavaScript. These technologies offered similar functionality with broader browser support, better mobile device compatibility, and didn[SQ]t require a separate plugin. Maintaining a proprietary plugin like Silverlight became increasingly difficult and less cost-effective for Microsoft. Therefore, support for Silverlight was officially ended in October 2021.

In short, you won[SQ]t find any new Silverlight applications being developed, and existing ones are likely to be obsolete or transitioned to newer technologies.