Szechuan Pepper is a peppery spice used frequently in food from Sichuan provence, China, but also in the cuisines of Japan, Tibet, and other countries in Asia. It has a slightly lemony aroma, and has a reputation for creating numbness in the mouth (very useful when eating really spicy foods like Mapo Doufu).
The flavour and spice are in the husks of the seed, not the seed itself. The seeds are hard like little stones, and are discarded before roasting and grinding the husks.
Szechuan pepper is one the ingredients in Chinese five spice.