Rubber Wood is the component of Rubber Trees. It can be extracted into 1 Rubber. Sometimes, Rubber Wood blocks have a Resin Hole (see image to the right). Right clicking the spot with a Treetap will produce 1-3 Sticky Resin, which can be smelted into 1 Rubber each in any furnace or extracted in an Extractor into 3 Rubber.
Be aware that right clicking an empty (dark) Resin Hole may produce extra Resin, but is very likely to permanently remove the Resin Hole. Therefore, it is advised to be careful when extracting Resin.
Rubber trees are rare trees. It may take a while to find any. They are, however, much easier to find within Swamp Biomes. They don't grow at all in Desert Biomes, so don't waste time looking for them there. Rubber trees have dark colored wood, like Pine, with bright leaves similar to Birch, and have a unique 'wing' in the leaf pattern. A screenshot of the tree and distinctive leaf pattern can be seen here: imageshack.us
Rubber Wood can either be extracted for Rubber, or turned into normal Logs or Planks.