My focus is on traditional breeds, as compared to commercial hybrids. I know they are convenient for backyarders, and often produce eggs well for a year or two, but the traditional breeds have value historically and genetically. Don't overlook them. Traditional breeds carry important, irreplaceable genes. They may have natural immunity to disease or be especially well suuited to hot or cold climates. Their genes may come to the rescue of the poultry industry of the future.
Breeds are the repository of genetic diversity in domestic animals. A breed has a unique appearance, productivity and behavior. They breed true, which means that when they are mated together, their offspring are predictably like them. They can reproduce themselves. Hybrids can reproduce, but their offspring will be a mishmash, mongrels.
Breeds are a package deal, not a collection of individual traits such as comb type and body conformation. We cannot know all the traits that comprise a breed. To lose a breed is to lose the entire unique genetic package. Breeders who raise traditional breeds always have extra birds they don't want or need. They have minor faults that aren't a problem to backyard chicken keepers, such as crooked comb or the wrong color legs, traits they don't want in their breeding pen. The birds are healthy and make good layers.
They give their keepers a chance to learn about the breed and be part of breed conservation. Most feed stores include traditional breeds among the chicks they sell, or you can get them from a poultry show. Tyler Danke, who started this thread, sells them at his Purely Poultry. You'll have more reasons to be proud of your fluffy Cochins or five-toed Dorkings than production layers.