Barbecue is different from all other cooking styles because it utilizes smoke. Depending on the material used as the heat source, different types of smokes are generated. As a result, the flavor of the cooked food gets influenced significantly by the heat source.

In the very early days, dried hardwood logs were used as the only source of fuel for all types of grilling and barbecue. The heat used to be the main factor responsible for cooking the meat. On the other hand, the smoke imparted a mouth-watering flavor that is considered to be the essence of BBQ.

Most of the modern day smokers and grills use gas or charcoal as the heat source. However, some still use electricity and wood pellets. These smokers get aroma and flavor with the addition of different forms of wood such as chunks, chips, pellets, logs, briquettes, and sawdust. Smoke can also be generated from spice, protein, and sugar laden meat drippings.
In general, it has been observed that hardwoods from nut-bearing and fruit bearing trees are the best as smoke woods. According to my own experience, the flavor imparted by fruit woods tends to be that of lighter smoke. A much stronger flavor of smoke is produced by the nut woods. I have seen many a times that barbecuers end up using whatever wood is abundant in their area. However, opting for the most appropriate smoke wood is one of the most critical decisions. As discussed above, each type of wood brings a flavor of its own to fish, pork, beef, seafood, and poultry. As a result, certain types of woods are considered to be the perfect match for certain types of meat.
Mentioned below are some commonly used smoke woods and their population applications.

Alder: Alder offers a mild, sweet flavor that is not overpowering. Produces best results when used with fish. Also, works quite well with pork and beef.
Apple: Comes with a fruity aroma, and is the preferred smoke wood for poultry and pork, especially for pork ribs.
Cherry: Its subtle, aromatic flavor works extremely well with chicken. Cherry wood cant darken meat so be carful.Cherry wood also goes well with pork and beef.
Guava: It has a semi-sweet flavor, and can be used with fish, poultry, lamb, and pork.
Hickory: This is probably the most frequently used smoke wood because its strong flavor complements all types of meats.
Maple: Maple wood goes particularly well with poultry and pork.
Oak: With a medium smoky flavor, it works well with almost any barbecue meat.
Always keep in mind that there are certain types of woods that are not at all suitable for BBQ. Some examples are cypress, cedar, eucalyptus, elm, pine, liquid amber, fir, redwood, sycamore, and spruce.
The table below will help you decide what woods to use for smoking foods

Beef  Pork  Poultry  Lamb Fish  Seafood
Alder                 Yes   Yes     Yes
Apple                Yes   Yes
Cherry              Yes   Yes     Yes
Guava               Yes   Yes     Yes         Yes
Hickory            Yes   Yes     Yes         Yes     Yes    Yes
Maple               Yes   Yes
Oak                   Yes   Yes     Yes         Yes     Yes    Yes
Grapevines      Yes   Yes     Yes
Birch                Yes   Yes
Lilac                 Yes   Yes
Walnut            Yes   Yes      Yes