The main difference are that key limes are a LOT smaller.  To get a quarter cup of juice for this recipe, I had to squeeze about 12 limes.  The other limes (known as Persian limes) are larger and have a thicker skin. Key limes are known for being more aromatic and a little more intense in flavor but less tart.  Sometimes they are hard to find if they aren't in season.  For the purpose of this recipe, feel free to substitute a Persian lime. Also, since the Persian limes do have a thicker skin, I used those to get my lime zest for the salad.  I think it would have been kind of hard to zest the key limes!