Specific Gravity
The density of wood is the amount of mass it contains per unit volume, usually measured in grams per cubic centimeter (g/c3). At room temperature, pure water has a density of 1 g/c3, so the density of materials like wood can also be described in terms of specific gravity, the density of the wood divided by the density of water. Wood that has exactly the same density as water will have a specific gravity of 1; if it's 1.2 times as dense, its specific gravity will be 1.2, and so on.
Floating and Sinking
Wood floats if it is less dense than water -- in other words, if it has a specific gravity less than 1. Most woods are indeed less dense than water, but some are actually more dense and will sink. Desert ironwood, for example, has a specific gravity of 1.15. Wood from the pau d'arco tree, the lignum vitae, the knob-thorn, the quebracho and the ebony tree are also more dense than water.
Pores
Wood is made primarily of plant cell walls composed of lignin and cellulose; these structures have specific gravities of about 1.5. Numerous pores in the wood, however, cause it to have a much lower specific gravity than the cell wall material, so that even the densest woods have specific gravities below 1.4. Specific gravity of wood can also vary somewhat within a species depending on the part of the tree from which the wood was removed, the tree's location and genetic factors.
Considerations
As long as the wood has a specific gravity greater than 1 it will float. It's important to note, however, that the density of wood also depends on the water content. Specific gravities for wood are always measured for oven-dry wood, which has been dried so it contains less moisture. Moisture-laden wood has higher density. The difference will only be important for floating or sinking if the specific gravity of the wood is very close to 1.