Adobe Flash (formerly Macromedia Flash) is a highly widespread mutlimedia platform that has, for many years, been the dominant means of enriching web pages with interactive content, animation, and video. Common applications include advertisements, video animations, and games (though, increasingly, gaming applications are abandoning the platform).
Flash's long-running success has lied in its combination of ease of use, interactivity, and accessibility. Because it supports bi-directional streaming, and because it reads user input via a variety of mediums (most commonly, mouse movements or clicks), Flash provides a relatively simple means of crafting interactive content, without requiring users to pay for or download any additional services--hence the platform's popularity among advertisers and web-based game developers.
How Flash Works:
Flash works by manipulating two forms of graphics, vector and raster, to give life to still images. A bit of background: Vector graphics utilize geometrical forms (ranging from points and lines to more complex shapes and polygons) to create computer graphics via mathematical parameters. Raster graphics, also known as bitmap graphics, determine the arrangement of pixels used to constitute an image. Flash changes the parameters of vector and raster information based on time-based commands and user input.
Flash for Mobile Devices:
Apple broke with convention in 2007 when it declared that it would not support flash in iOS, its mobile operating system. Citing a number of concrete issues with the platform, and with Adobe's handling of the transition to mobile devices (most importantly, its lackluster security record), Apple set into motion what now appears to be a conclusive trend among handset developers. Indeed, in late 2011, Adobe itself announced it planned to halt its development of the mobile Flash platform. It's place has largely been filled by HTML5.
That said, mobile Flash has been supported by all major versions of the Android operating system (since 2.2 Froyo), by elements of the BlackBerry OS, Symbian, Palm OS, and webOS.