I'm over 60% done with the game but I still have 0/50 "Film Reels" (as shown in the Statistics page). What / Where are Film Reels?
Film Reels are well-hidden collectibles, that are scattered around the game.
What you are looking for
You could spend hours and hours of searching and looking the 50 reels, or you could use this guide that shows the location of all the films.
They are the hidden collectibles.
You can find a map of them here. http://socialclub.rockstargames.com/games/lan/checklist.html
Film reels are little golden circles (film reels, more or less!) that you can find in various locations scattered around the city. If you can, login to Rockstar Social Club, and there's an interactive map to all of them (along with every other pickup in the game, practically) you can use. I know on consoles, the map is tied to your save game, so it will further tell you where the ones you're missing are.