Ameraucanas and Easter eggers have a lot in common, but Ameraucanas are bred to the APA breed standard.
Ameraucanas have pea combs, beards, muffs, black/slate legs, and lay blue eggs. They come in one of several specific colors (black, blue, blue wheaten, brown red, buff, silver, wheaten and white). Within their genetic colors, they breed true. They can be shown at poultry shows and are consistent in their appearance and other traits. They also occasionally come in "project" colors, which are colors not recognized by the APA, like splash or lavender.
Easter eggers do not breed true. Many of them lay brown or green eggs. The muffs, beards and peacombs are optional and may or may not be present. Their legs are usually green but may be other colors and they have no consistency in their colors or shape. They can not be shown in APA shows and cannot win awards.
From a casual perspective they may be very similar. But one is a breed with consistent traits (Ameraucanas), and the other is a mixed group of chickens with a few inconsistent traits that overlap (Easter eggers).