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. They therefore rarely have prizes or entry fees.
Film Festivals can be one or the other or somewhere in between.
A Video festival means that only video productions are allowed. Always label your tapes clearly as they can get separated from your other materials. If you can print a label on your computer using the 'Avery' type, pre-cut, stickers. Its always worth considering producing a VHS cover for your video with a DTP package. The extra cost of sending a boxed VHS through second class post is little.