Since 1926 all pedigree dogs in France have been named based on the letter of the year.

The Société Centrale Canine (the Central Canine Society) is the kennel club of France. Since 1926 the SCC has had a naming convention that all pedigree dogs born in the same year are given a name starting with the same letter – the letter changing each year. So all pedigree dogs born in 1926 had names that started with the letter A, in 1927 they started with B, and so on. The intention was to simplify the work of dog genealogists tracking the lineage of pedigree dogs in the country.

Over time some letters were removed from the system because of how few French names begin with those letters. The letter Z was omitted from the system at the beginning and in 1973 K, Q, W, X, Y were all removed. This left a 20 letter system where, when you meet a fancy pedigree dog, you know exactly how old it is by its name.

Added info: this French naming rule only applies to pedigree dogs, not all dogs. Pedigree dogs are dogs whose lineage has been recorded. Mutts, adopted dogs, shelter dogs – none of these are restricted by the naming convention.

As for the difference between purebred and pedigree, purebred dogs are those whose parents are of the same breed. Pedigree dogs can be purebred or can be mixed breed, but whatever their lineage their genealogy is recorded.