You know, neither am I. I am a bit afraid sometimes if I don't feel well prepared but, nah, that makes me rather feeling annoyed.
The problem is that one becomes nervous, which is a usual reaction. The problem starts when this nervosity takes over the control in some kind. One is way too fast when speaking or writing things down although they have ages of time still, or they stop logical tinking, can't think logical anymore and begin to Huddle as we say in Swabian here.
I think Prüfungsangst is when your nervousity becomes so high that it hinders you from working well in your exam, simple as that. Sadly, I always have that, it's just a little less in oral exams.