Its all about the smoke, and different types of wood will provide different types of flavor. I have personally tried most varieties of wood while smoking. The majority of wood that I use is chunk form, although smoke wood can also be purchased in chip form and pellets. Chips would be used as a short smoke, and pellets can also be used for a short smoke or a cold smoke.
Hickory is my favorite, I love the smell of burning hickory. I use it when I smoke pork butts, pork loins and cashews. I have also used it for smoking chicken on occasion and it tastes good.
Cherry is the next wood that I use a lot of. Cherry seems to give the best smoke ring from the Weber Smokey Mountain. I use it for cooking chicken, and turkey. I have also used it for smoking almonds and I have used it for brisket. It is my favorite wood for smoking cheese.
Apple wood gives a lighter sweeter smoke flavor. I have used it on chicken, and pork loins
Mesquite is popular for beef. I have used it smoke brisket, and it gives a strong smoke flavor.