Flash files is a topic that seems to cause heated discussions between web site designers, webmasters, and site owners: They usually either violently oppose them, or are passionately in love with them.
Flash is a widely used graphics file format used for animations; the animation designer can provide a degree of user interaction with the person viewing the animation. Properly done, a Flash animation can be quite striking and compelling. Poorly done, visitors can't seem to hit the "back" button quickly enough to flee the experience.
Most web surfers have encountered at least one site that uses a Flash "introduction page": Frequently you are forced to view a very pretty, very lengthy animation (which usually imparts little information about the site), before you are allowed to see the actual site. Broadband customers may or may not be fazed by this, but dialup visitors scramble to get away from the site as quickly as possible because it takes too much time to download this single file before they can see a site they may or may not care about.
Turgid and lengthy Flash "movies", introduction pages and the like are BAD because they drive people away from your site. Concise animations of 3D product displays, graphic depiction of otherwise difficult to describe processes, embedded graphics that don't hinder navigation - these are all GOOD uses of Flash because they enhance the visitor's experience.