Mexican, slightly smaller, very aromatic, and with a particularly bright green skin;
Key lime, which have a paler skin, a high juice content and a strong flavour.
Limes are available all year round, when buying them look for unblemished, firm limes that feel heavy for their size as they will be the juiciest. If you intend to use the zest, buy them unwaxed (shops should state this clearly). If you can't find unwaxed limes, scrub the limes thoroughly before zesting.
All the best Jan