MacromediaFlash is the de facto standard for animation or interface design on the Web, well ahead of even HTML tricks like DynamicHTML?, or even JavaApplet?s. It comes standard on [98.3%] of computers these days, as it is bundled with the major browsers. It's development environment is easy to use, powerful, and gives graphic designers that warm fuzzy feeling they miss in streaming, random layout HTML. It is also curries extra favour by being efficient. It is a very tight binary stream consisting mostly of cheap-to-encode vector graphics.
In the early days of the Web, Macromedia had put Shockwave onto everyone's desktop by similar bundling deals. [Shockwave] was very similar to Flash, being also a binary stream of multimedia content, though it leans toward video and bitmapped graphics rather than vector graphics. However, a struggling start up called FutureWave pulled a major coup in 1996 by convincing Microsoft and Disney to use a competing technology, FutureSplash, for their upcoming new ventures. Macromedia decided it was better to join them than fight them, so it bought FutureWave in December 1996, giving rise to MacromediaFlash 1.0 and never looking back.
To counter their monopoly, the W3C has created an open XML standard called ScalableVectorGraphics (SVG) to compete with closed, binary Flash. Ironically, this comes mostly at the behest of Adobe, who had earlier skipped the chance to buy Flash. Macromedia is [opening] their Flash specification in response, as well as [providing] an encoder API and even the source code to the Flash player to interested third parties.
Even though Flash exists on almost every computer, it's not readable by everybody. Flash has severe accessibility problems as well as usability problems that are symptoms of its graphic designer background. They are working on it, though. [(accessibility)] [(localization)]
Naturally Flash is evil, yet Flash is amazing.