At least one of the panic episodes must be unexpected and not caused by a situation in which one is the focus of attention. Fear of death or losing control or losing sanity generally accompanies these attacks. It's important to distinguish between general feelings of anxiety and panic. In a panic attack anxiety and fear rapidly surge to a very uncomfortable point, then subside after 10 to 30 minutes with some symptoms possibly lasting for several hours.
Anxiety tends to be more generalized and stable in nature.
A panic attack can be understood as the body reacting naturally to a panic situation when there is no real danger present. You might be browsing the local bookstore for a new cookbook when suddenly your body goes into a fight or flight reaction for no apparent reason. Hence the apprehension component of the disorder. If another attack occurs the apprehension intensifies. It can become an endless cycle, apprehension triggering panic, panic triggering increased apprehension.