may sound like a silly question, but how is a certain breed of dog made? and what then makes it a purebreed?

Not silly at all - some breeds have been evolved to fit a purpose and dogs who fitted that purpose best would have been selected for breeding - gradually this would have resulted in a certain common set of characteristics which would reporoduce itself through resulting generations and become a 'breed'. Most of the working breeds evolved in this way. Some breeds have been deliberately made by crossing  other breeds ( The Eurasier was made by crossing a Chow, Samoyed and Keeshond to produce a quiet spitz breed) several generatiions would need to be proven to breed 'true to type' in order to be accepted as a breed.

Interestingly if left to their own devices most feral dogs will gradually assume a generic type as well - think of the Caanan dog, Basenji, Dingo etc this appears the most effective dog 'blue print' for survival .

I'm sure others will be on here with better explanations than I can priovide ;).

Yvonne