The wood block is literally a block of wood, but typically hollowed out to create a cavity that could serve as soundbox. Modern woodblocks are also made of artificial materials such as plastics. Being stressed during the manufacturing process, or natural growth of the tree, the wood block has a single pitch, although different harmonics might be added to the base soundwave by striking the block at different places.
Source: http://en.wikipedia.org/wiki/Wood_block
http://www.youtube.com/watch?v=Rgc839e_wVc