Rubs come in either dry or wet form and are used to flavor meat of all types when Barbecuing. You can buy them pre-prepared or make your own. After the rub is applied it needs to marinate for awhile to ensure that the flavors soak into the food. Rubs can also be used when baking or roasting or any dry heat method of cooking but it is not recommended to be uses in boiling or any type of cooking that contains a lot of fluids.
Many people that enter BBQ contests won't share their rub recipes as they claim it has secret ingredients and combinations to bring their meat to just the perfect flavor. Some rub recipes have been passed down through the generations from family to family. Rubs consist of many different spices and variations, its basically what tastes best to you.