Ameraucanas (sometimes incorrectly refereed to as 'Americanas') are a breed of chickens related to the Araucana but are definitely not Araucanas, and may God help you if you ever get them confused in front of a serious hen breeder. The confusion isn't helped by the fact that both have similar names (Ameraucana is actually a portmanteau of 'American' and Araucana), both have the blue egg laying gene, and both have peacombs.
Unlike the Araucanas, though, the Ameraucanas have froofy feathered rumps (Araucanas are tail-less) and lack the Araucana's ear tufts.
If you're going by the standards set forth by the American Poultry Association, official Ameraucanas are rare. They aren't as rare as official Araucanas, but are still pretty hard to come by and can only be gotten through breeders.
Ameraucanas are sometimes referred to as Easter Egg chickens. That isn't actually accurate as "Easter Egger" is not actually a breed; it's just what people call anything that has the blue egg laying gene but doesn't meet to the APA's standard for an actual Ameraucana/Araucana. Nine times out of ten, if you go somewhere and see them advertising Ameraucanas, they're actually just going to be Easter Eggers.
But, dude, who cares? BLUE EGGS. (Also, sometimes white eggs, but the blue egg gene is dominant over that one, and white eggs are boring anyways.)
Funny thing about the eggs; depending on the chicken's lineage, you'll wind up with eggs that range from pastel blue (considered to be true Ameraucana eggs), to slightly darker shades, to actual green, the reason being that green eggs are actually blue eggs tinted brown slightly by some finagling genetic code because daddy wasn't actually an Ameraucana, just a brown-laying Plymouth Rock. The way to tell how bad the tinting is is to compare the outside of the eggshell with the inside.
The recognized APA colors for Ameraucanas are: Black, Blue, Blue Wheaten, Brown Red, Buff, Silver, Wheaten & White. Lavender is also a popular breeder color, but it isn't officially recognized yet.
Information gotten from, among other places, The Ameraucana Breeder's Club