White peppercorns come from the same berries as black peppercorns, but they're vine-ripened longer, and the black shell is stripped before drying. Their flavor is sharp, floral, almost winy, and hotter than black pepper. After their initial pungency has dissipated, they leave a glowing warm sensation in the mouth. White pepper is the pepper of choice in many places, including Indonesia, Malaysia, and Thailand. In classic Western cooking, it's often used in pale foods, such as white sauce or mashed potatoes, because it doesn't "mar" the food with black specks.
Black pepper can substitute, though you may need more of it.
Try to buy peppercorns from a source with good turnover. Sarawak peppercorns are a very high-quality white pepper.
Use whole to infuse liquids with flavor; otherwise crush or grind the peppercorns before using.
Store them away from heat and light (like any spice), and use them within a year.