Apart from natively supported types such as images, browsers require what is known as plug-ins to load and display multimedia content. Some of the commonly downloaded plug-ins by Internet users are the Adobe Flash player, Sun Micosystems Java, Microsoft Windows Media player and Apple Quicktime player. Take a look at the pervasiveness of plug-ins statistics in browsers by Adobe.
Adobe Flash player is the most downloaded plug-in for web browsers. It will be good if we can harness some of its features to boost the interactivity of the site.
As Adobe Flash is a commercial product, you will need to purchase a legal copy. You will also need to learn how to use ActionScript in Flash (unless you are using Flash just to do simple animation).
A comparison between JavaScript & Flash is provided in the report Flash vs Ajax
To summarize the article, AJAX is on the rise, but Flash still offers some good features that cannot be achieved by JavaScript. Flash allows for audio, has little compatibility issues between browsers or devices and also caters to video content streaming.
Whether to use JavaScript or Flash, it depends on what you intend to achieve. The chart in the above report above will be a good reference to help you make your decision. For simple interactivity, we usually use JavaScript since it is easier, does not require third-party software, and is free.
If you use Flash in your site, be sure to have a detection mechanism to check if Flash is installed in the user's browser. The script can be downloaded from Adobe Flash detection kit page.