When trying to get at the technical core of what the FBI is asking of Apple, this researcher/hacker/forensic scientist/? seems to have the explanations.
Might I suggest you start here and here and a Summary
Really you should just go read the last 15 or so posts.
This is a controversial topic, but at the core of the debate is a tremendous amount of misinformation and purposeful ambiguity. Fight the FUD (fear, uncertainty and doubt).
Source: Zdziarski’s Blog of Things