Well... can create Flash files with Macomedia Flash MX.
U can put them on ur website, the files are called .SWF.
U need a flash plugin to play them. If ur browser has a flash plugin from macomedia installed u can open any .SWF file in ur browser.
Flash can pretty much do anything. It has its own language called actionscript. IT can actually be used to write applications as well. Probably not the best language to write apps but still possible.
to render or display flash content, you'll need to install a browser plugin. you're typically prompted if you don't already have it installed. flash is probably one of the most efficient way to display audio/video on websites without having to worry about different operating systems and their typical media players...its very popular in advertising for this.