Key limes are smaller, juicier, and more acidic than Bearss limes (a.k.a. Persian limes), with a sharper, more bitter flavor that is highly prized by those who like them. Key limes are also known as Mexican limes or West Indian limes. While you may find them at grocery stores looking as bright green as an emerald, key limes actually turn a much more yellow shade (as pictured above) when ripe. These yellow, ripe key limes are juicier, with a fuller floral flavor than their under-ripe counterparts.
Key limes are named after and associated with the Florida keys, where they are used to make the famous key lime pie, a concoction of key lime juice, egg yolks, and sweetened condensed milk that, traditionally, depending on the "souring" power of lime juice to "cook" the egg yolks and thicken the pie filling (because of concerns about food safety, key lime pies are usually baked now) and topped with a sweet meringue or whipped cream.