Here's a quick and dirty run down of the differences between generations of fighters:
Fighter aircraft - Wikipedia, the free encyclopedia
Both the Fitter and the Fencer belong to the 3rd generation. The Fencer being an all-weather variant. I guess the Fitter could belong to the 2nd generation along with the century series from US.
There's never a clear line on which one belongs where. Sometimes there are transition designs with features from across 2 generations.