Flash can be used to create interactive rich media content (including text,
photos, music, video, vector graphics, voiceover, and a wide variety of motion
graphic effects and animation) to be displayed in a browser. By producing
a compiled Flash movie and an html page which embeds that movie and uploading
both to a web server, the content is made viewable to a site visitor via a
Flash browser plugin, the latest version of which can be downloaded for free
here.
Flash can also be used to create executable files, to be played from a CD or from the desktop of either a PC or a Mac, without requiring any additional software (the Flash player is embedded in the executable).
In this class, we'll concentrate on using Flash to create online movies. (A Flash source file is also referred to as a Flash movie, whether it is actually an ad, a cartoon, a complete web site, a game, or some other Flash application). Flash is an ideal application, as we'll see on the next page, not only for entertaining or making a statement on a website, but for any online application involving user interaction, display of database content, need for resizable graphics (maps, eg), explanatory animation, custom navigation, or seamless/custom integration of video or sound.
To create a Flash movie, one works within the Flash IDE (integrated development environment), either drawing vector graphics on the stage or importing graphics and/or music and/or video from outside the IDE. When a file is imported, a copy is placed in the Flash movie's library (every movie has its own). Animation within a Flash movie is achieved by using a series of frames and keyframes, each with its own graphic/sound/video/text content. In each keyframe of the movie, appropriate graphic content is placed on the stage in the IDE, and sound and/or actionscript may also be added to the frame. This source file is saved as a file with extension .fla. To be viewable by someone else online, it must then be published to produce a .swf file (published Flash movie) and .html file (in which the swf is embedded).
The image below is a screen shot of a Flash movie under development. In it, you can see the timeline panel at the top, which shows that the movie (fish.fla) contains actionscript (indicated by the little 'a' in the actions layer), sound (beginning in frame 1 of the music layer), and graphic content in the fish1 and fish2 layer. An item in the fish2 layer is currently selected (the fish with a blue rectangle around it), and the stage content is that of frame 2 (where the playhead is currently pointing). The stage is the big area in the middle where graphic content for the movie is placed. At the bottom is the Properties panel, which shows information specific to the currently selected item on stage, or the currently selected frame in the movie (depending on whether an item on stage or a frame was last clicked). On the right is the Library panel, which shows that there are currently 3 items in the library of fish.fla: a sound file and two movieclips. All panels in Flash can be hidden by clicking on their title bar, unhidden by clicking again on the title bar, opened via the Window menu option, closed by right-clicking the title bar and choosing Close Panel, and undocked by dragging the dotted section on the left end of the title bar.

As with any web technology, there are some applications which are best done with Flash and others which aren't. For text and information-based sites, the ability to provide deep url links and search engine spidering that you get with an html-based site outweighs the advantages of the more robust user interactivity and resizability you can get with an all-Flash site, in my opinion. You can, however, include Flash content within tags of an html site, as is done extensively at this site.
Other Resources
Need more information?
Press F1 or choose Window, Help to bring up the Flash help panel from within the Flash IDE. Click the How Do I tab and choose Quick Start for some introductory tutorials on using Flash.
The same documentation with comments added by Flash users and Macromedia staff may be found online.
Flash Extenders
If you want to use Flash to create a projector file that will interact with files on the user's computer, you might want to check out these 3rd party tools, which allow you to produce standalone applications that can incorporate local file system interactivity with your Flash movie, create screensavers, create autorun files, etc: