This video pretty much sums up the basics. It doesn't really take into account some of the non-residual issues like coverage for animation writing, etc., but is a decent primer if you want to get a grasp on why writers felt the need to authorize a strike in order to protect the future of a residual based payout structure.
What can't be stressed enough is that a residual is only paid on earnings a studio makes. They are not arbitrary payments. A flop of a film that garners no public interest will pay a low amount of residuals as there's a lower demand for it in ancillary markets. A huge success, on the other hand, has lots of ancillary options and play and thus as networks pay to televise the film and consumers pay to re-experience it on DVD or cable - out of such newfound financial interest in the film a residual payment is also made to the writer. It's not really an added cost and anytime the studios pretend a residual is an added cost it's intentionally misleading.
This video is from www.unitedhollywood.com a really well done site that has tons of info and first person accounts regarding the strike. --JG