Fast bowlers

It has always bugged me how fast bowlers at international level are classified.
For instance, Jason Gillespie, Darren Gough, Wasim Akram etc were all classed as medium-fast.
Why is that? How do you distinguish between fast and medium fast?
The only bowlers I remember being classed as fast were Lee, Akhtar, Steyn, Johnson and Tait.
For instance, Jason Gillespie, Darren Gough, Wasim Akram etc were all classed as medium-fast.
Why is that? How do you distinguish between fast and medium fast?
The only bowlers I remember being classed as fast were Lee, Akhtar, Steyn, Johnson and Tait.