Jerk is used in both dry and wet forms. Dry rubs are ground spices rubbed into the meat immediately before it is placed on the
grill or in the oven, much in the way that
seasoned flour is rubbed into or patted onto a
pork chop. Wet jerk is used as a marinade, in which the meat soaks up the spices before cooking.
The original purpose of the jerk marinade was to help preserve meat as well as giving it a unique spicy flavor. Any meat can be "jerked;" pork was the original jerked meat, in the form of wild pig, but nowadays jerked chicken and fish recipes are very popular.
As with any food that has a long history, there is no one jerk recipe. Everyone has their own take on how jerk should taste and what it should contain. Common ingredients include chilies, allspice, and salt; there are often dozens more herbs and spices, including nutmeg, cinnamon, thyme, garlic, pepper, lime, onions, scallions, and many more.
According to foodreference.com, "Its origins date back to the native Arawak Indians traditional method of using Jamaican pimento (what we call allspice) to season and smoke meat.... Combine this with hot chilies (originating in South America and the Caribbean) with pirates bringing in a variety of new spices from both the old and new worlds; add salt and escaped slaves with skills at slow roasting in pits, mix well and you have jerk."