Flash Player is a versatile and helpful tool which is being made available for developers and consumers to test their content to ensure new features function as expected, existing content plays back correctly, and there are no compatibility issues. Adobe Flash Player 10.1 prerelease version of Flash Player 10.1 is now available for Windows, Macintosh and Linux operating systems.Consumers can try the beta release of Flash Player 10.1 to preview hardware acceleration of video on supported Windows PCs and x86-based netbooks. You can also help make Flash Player better by visiting all of your favorite sites, making sure they work the same or better than with the current player.
The Flash Player is available as a plugin for recent versions of web browsers (such as Mozilla Firefox, SeaMonkey, Opera, and Safari) on selected platforms. The plugin is not required for Google Chrome anymore since Google integrated the file into the distribution. Adobe states that each version of the plugin is backwards-compatible, with the exception of security changes introduced in Version 10
You need flash player to do following
view videos online on sites like Youtube
play online games on the web
play Facebook games like Farmville etc
online video conferencing in the web browser