I fly business class with my company and obviously get airmiles credited to my frequent flyer club. In the past have always flown economy when on holiday as business class is so expensive. I have started flying Premium Economy on holiday flights now and after finding this forum, I'm 'confused' about different fare 'classes' eg, Z, U, J, D etc. Can someone explain to me what these are, how to find them when a fare is displayed and are they the same for every airline or is each airline different?

Being a IT Software Engineer, I should hopefully be able to follow any logic that these classes may follow so feel free to explain in logical terms if it is easier.

Regards