Flash is a for making graphics move. It also has capabilities for creating still images, vector graphics, drawings and text, but the main draw of this software platform is for animations, be they on websites, games or even TV. Popular children's cartoons like My Little Pony, Clone High and Pound Puppies were all animated in Flash, for example, as well as countless addictive web games.
Flash looks a bit like any other graphics program, such as Photoshop, but it also has its own language, ActionScript, which allows a programmable aspect so graphics can move according to a pre-determined schedule. It has capabilities for bidirectional streaming in video and audio, and can also register and act upon mouse, keyboard, microphone and camera input, making it a very powerful tool. Flash used to be heavily used in web design for this reason, and shifting, responsive menus and sliders with transition effects were very common. However, its real staying power on the internet is in video/audio players, advertisements and games.
In order to play flash in their browser, a user must have Adobe Flash Player installed, but this comes free with many common browsers and OSs for many systems and devices, though with some notable exceptions for iPads in the past. Flash files are saved in a .swf format which is pronounced 'ShockWave Flash'. They are also often traditionally called 'movies'. These are used as plug-ins for web pages, and the mix of vector graphics and efficient program code means that these files can be smaller than equivalent bitmaps or video clips. That means less bandwidth and CPU power is used, and therefore a faster browsing experience is possible. However, due to incoming advancements with HTML5, etc., as well as web design trends turning away from Flash, it is not as common to find outside video/audio players and games as it once was.