Isn't it 'normal' to feel apprehensive about certain situations - ones that are a bit scary or very important etc?

It is a natural and even adaptive response - 'nerves' give many people a bit of an edge in performance in many areas because you are that bit sharper and more aware.

'Disordered' is when the feelings are less rational, things are catastrophised, it occurs over small things instead of something important like say a job interview, and the anxiety interferes with everyday functioning and ability to complete work, family, social tasks etc?