Easter egger: is a mix between an Ameracauna and another bird. It can also be a mix between an Auracauna(spelling?) and another bird. Easter Eggers come in any color you can think of and have only breed generalizations, although they're not really a breed. *most* have muffs and beards, green or willow legs, a peacomb and will lay green blue or even pink eggs. They are not recognized by any poultry association as a breed. Easter Eggers are generally friendlier, lay more often, and are les flighty than their pure bred ancestors. Many hatcheries will (whether on purpose or not) sell Easter Eggers as "Americana or Aracauna" although they are only Easter Eggers.
Ameracauna are pure bred birds with a peacomb, muffs and beards and black/slate legs. They only come in a very select few colors and will lay blue eggs. If they do not have all of these qualities, they are not considered true Ameracauna. The only place you can get pure bred Ameracaunas for the most part is from private breeders and not hatcheries. I know of one hatchery that claims to have wheaten Ameracauna, but I don't know the creditibility of that. You can check out the "Ameracauna scrapbook" for the acceptable colors they can come in.
Aracaunas are rumpless(tail less) birds with ear tufts. Ear tufts are different than muffs and although I don't know much more about them, I feel they only come in certain colors too. They also lay blue eggs.
Are you looking for a specific breed or just a chicken that lays colors other than brown and white? If you wan sky blue eggs, I highly recommend Cream Legbars. They are auto-sexing (meaning you can tell the sex at birth with a very high accuracy) and lay a beautiful sky blue egg. So if you live in an area that does not allow roosters, being able to tell the sex at birth is probably the Cream Legbar's best selling point.
Or if you want a true olive egg, try Isbars (pronounced "Ice-bar"). The blue variety are gorgeous and they lay nice olive eggs. Experienced breeders can tell Isbar sex at birth, however, I have never been able to do that. So if you can't risk getting a rooster, definitely go for the Cream Legbar over the Isbar.
There's nothing wrong with Easter Eggers either. They really are beautiful birds. I just prefer rare and unusual chickens.
I was just reading through the list of different breeds with my son and we both thought it would be good to have some EEs. And since it turns out that there's someone selling some not far from me, I figured I'd find out more about them.
If you are only looking for pretty blue and/or green eggs, you needn't really worry about the difference. All of them should lay blue or green eggs. Just find some pretty pullets of any of those varieties that you like and enjoy them.
The only real reason to worry about the differences is if you plan to breed and/or show them.