Persian limes - the most common commercial variety - are a small, round-to-oblong citrus fruit that can vary in size. They have a thin, smooth, glossy, rind. They are picked and shipped when bright green, although they will turn yellow if left on the tree to ripen. The flesh is pale green and typically seedless, or nearly seedless. The flesh is juicy and aromatic with an acidic, sour flavor. 
Persian limes have a thicker rind than Key limes which allow them to keep longer. They're predominantly used for juicing, zesting, and being served as a garnish.