Short form:
5c is not 'cheaper' the C seems to stand for Color, you get the choice of white, pink, yellow, green and blue. Naa sad siyay mga matching cases in similar colors plus a black one. Internally, it's just the iPhone 5 transplanted into that new body. Speaking of, the now-familiar Aluminum/glass body of the iPhone 5 is replaced by a polycarbonate body much like a Lumia phone (Think the Lumia 720) pricing isn't much cheaper though, 16GB model is $549 and 32GB model is $649. WHEW.
5s is a lot more Secure though. Introducing the TouchID sensor which enables the utilization of the user's fingerprint as a means of unlocking the phone. Additionally the TouchID can also be used to authenticate purchases on iTunes or the AppStore instead of keying in your password. Other new things is a new body color - Champagne Gold. White and Black models are now renamed as Silver and Space Grey. Internally everything is bumped up, processor is up to 64-bit, Camera now does 120fps SloMo video, better lens and sensor.
iOS 7 will be available for iPhone 4/4s/5, iPad 3/4 and iTouch 5 by Sept 18th.