I was going to say a pure bred is a loaf with no added chemicals ...but then thought better of it !!!
You are right though , a pure bred is , for arguments sake , a light Sussex hen or hens mated with a light Sussex cock bird. Much the same as a pedigree dog really . A weimaraner mated with another weimaraner gives pure weimaraners. Mate the same weimaraner with a spaniel and you just get a mutt , lovely though they may be.
It is the same with chickens , pigs , sheep or horses whatever . It is breeding within the type/breed and not out crossing with another type/breed.
A pedigree is a traceable lineage of the type /breed in question , whereas a pure bred is kept as pure as the pedigree , but without the paper work to prove lineage , as a rule .
To confuse matters , outcrosses are performed sometimes , but that is usually for health reasons ie if stock numbers are very low , as in a very rare breed , but this should only be done by someone with a good knowledge of breeding genetics and the type/breed in question.
Sorry for the long winded reply , but hope it helps explain a bit .
cheers
Russ