Wood chips are easy to find and are about the size of a coin. They burn quickly, so be sure to add more chips a few times during the cooking cycle. Chips are good for shorter cooking times. They begin to smoke more quickly than wood chunks, so choose wisely. If you're going for long, slow cooks, chunks or logs are your best bet.
Wood chunks don't burn as quickly as wood chips, so you get a slow, steady smoke. A chunk or two may be all you need at the start of the cooking cycle. Wood chunks produce some of the most desirable conditions for smoking.
They say real pitmasters cook with logs. It's a true art form to get the signature flavor you want out of the right wood. But when done right, you'll never go back.