Well are you really asking about “parakeets” a catch all name for most long tailed birds like Alexandrian Parakeets, Indian Ringneck Parakeets, Princes of Wales Parakeets, etc., etc. There are quite a few more in this category.
OR are you really asking about budgerigars which come in only one species? Since you are asking your questions on the budgie list it will be assume you are really asking about these birds. They go by many different “common” names such as American Parakeets/Budgies, Wild type Budgies, Bush Budgies (if you are in Australia), Pet Budgies, English Budgies and Exhibition Budgies. American pet stores are so far behind the times to properly identify these birds it is beyond sad always listing them as “parakeets.”
The English or Exhibition budgies come from a group of birds that has been carefully bred for over 200 years to reach a man made stylized look. These birds are MUCH larger in size, have a longer and heavier feathering than there wild cousins. If you would like to see examples of these birds you can visit the Budgerigar Association of America’s web site
http://www.budgerigarassociation.com Rarely are these birds found in pet stores and the ones you do usually these will be of the poorest quality available. For the same money you pay at a pet store you can get a bird that is 3 times better quality through a breeder.
Going through a pet store, usually a large chain pet store the “American” budgies you find there will be bird mill bred birds. These birds are produced along the same principles as puppy mills. The birds are bred past exhaustion (to death). The resulting offspring starts to shrink and health is compromised. These facilities restock breeding birds from within. So smaller birds are again bred past exhaustion (to death) resulting in smaller less healthy birds. In some areas I have seen “American” budgies raveling the size of finches. These pet stores ARE counting on the shorter lived birds in hopes you will return sooner to replace your pet.
It is better to buy your birds through a breeder to cares about his/her breeding stock. They are out there. You can even list an ad on the classified section looking for budgie breeder in your state/area.
You should look into reading up on these wonderful birds. If you local pet shop does not carry a good supply of budgie books try order from
http://www.abebooks.com I have always had the best results oredering from these people. The books +shipping have cost less than many books from my local book store. I have ordered books as far away as England and Ireland. You can do a search for your birds by subject which is VERY helpful!