Scripts, applets and plug-ins add dynamic content to the web.
Scripts are used for all aspects of web content. They can write all or some of the content of a page as it loads, or they can write hidden content to be displayed through some user interaction that, itself, can be implemented in scripts. Scripts are used to validate entries in forms and to add highlighting and other text effects.
Applets and plug-ins usually bring up new windows and offer essentially the same collection of accessibility concerns.