Firstly you send a preview copy of your production for the festival organizers to watch. This would normally be a VHS copy. Sometimes there is an entry fee which is sent with it - this is normally a small amount to cover costs. If they like your film they may request that you send a better format than VHS or the master or a film print so that the best quality will be shown.
Some events also like to have additional items such as photographs or a synopsis.
Film Competitions have prizes to award in various categories. The prize fund and the number of prize categories vary a great deal. These change in nature with the event and in keeping with a major sponsor, location, arts funding etc. Film Screenings, Presentations or Showings are more of a chance just to get your work seen by other film makers. They are a good chance to meet people if you can get there.