Apple released a white paper today detailing much of the security infrastructure in iOS:
Apple designed the iOS platform with security at its core. When we set out to create the best possible mobile OS, we drew from decades of experience to build an entirely new architecture. We thought about the security hazards of the desktop environment, and established a new approach to security in the design of iOS. We developed and incorporated innovative features that tighten mobile security and protect the entire system by default. As a result, iOS is a major leap forward in OS security.
Absolutely worth a read, especially as it is so readable. While this may be useful to security experts, most of it reads as though it were written for it to be understandable to you and I.