Regular limes (also known as Persian limes) are larger and more oval-shaped with a kelly green hue when ripe, while key limes are smaller and more round with a yellow-green shade when ripe.  In terms of flavor, regular limes are a bit more bracing and acidic, whereas key limes are milder and have more floral undertones. Key limes can also be a bit more difficult to find and may only be offered seasonally depending on where you live. Feel free to use either in this recipe for lime cookies!