NSError and NSException

NSError is designed for non-fatal, recoverable errors. The problems that are designed to be captured by an NSError are often user errors (or are errors that can be presented to the user), can often be recovered from (hence -presentError: and NSErrorRecoveryAttempting), and are usually expected or predictable errors (like trying to open a file that you don't have access to, or trying to convert between incompatible string encodings).

NSException is designed for potentially fatal, programmer errors. These errors are designed to signify potential flaws in your application where you have not correctly checked the pre-conditions for performing some operations (like trying to access an array index that is beyond its bounds, or attempts to mutate an immutable object). The introduction to the Exception Programming Guide explains this a little bit