flash

Flash is a proprietary format for delivering dynamic and interactive web content.  Owned by Adobe (formerly Macromedia Flash), Flash content must be displayed in a virtual environment called a 'Flash Player', which traditionally has taken the form of a web browser plugin--however there are stand alone and embedded players as well.  

Features include raster and vector graphics, audio/video streaming, and a built in scripting language to make it dynamic.  

Common Uses

The most common use of Flash is probably web advertisements, however in recent months this is becoming closely tied with Flash's use as an embedded media player within a webpage (i.e. youtube). 

There are also numerous games written in Flash, and more and more functional applications are being released in Flash format as well.   

Many websites also use Flash for dynamic presentation of web content, such as animated intro's to websites, or animated menus.  Some developers even release entire web sites written in Flash.   

Using Flash 

In order to use Flash or view a site with Flash content, one must have a Flash player.  The official Flash plugin is available for most popular (graphical--sorry Lynx) web browsers, but there are several third party players in various stages of functionality.   

Making Flash files is a little more tricky, though Adobe would be happy to sell you an IDE to do it with (or even let you have a thirty day trial).   

Versus Java

Typically, Flash plugins are smaller, lighter and faster than Java environments, contributing to its increasing popularity.  

Both are proprietary platforms, limiting control of licensing to their respective owners, and requiring trust in the provider (as source code cannot be verified for security and reliability).   

Criticism

Being proprietary, Flash is controlled by one company (unlike xml,html,etc), and subject to licensing terms provided by Adobe (making it ineligible to be packaged in Free Software projects).  

Other criticism includes accessibility concerns, and security concerns (as vulnerabilities in Flash can introduce the possibility for execution of malicious code).