A dead tree. A chunk of wood that can be turned into many things, most obviously Lumber. Logs are acquired from Logging Camps, or by foraging. They are one of the six basic building materials, and the most restrictive of these, allowing only one level. If a supply of logs are available, they may be used effectively for buildings that cannot be upgraded, such as Warehouses or (ironically enough) Spacecraft Factories.
Most of the time resources are not so scarce that the production of logs by a city is necessary, as Metal and Plastic may be used to make anything creatable by Carpenters. But it may be useful to augment the production of certain goods (such as shovels and hand-weapons) with wood, as this increases the amount of raw material available to other industries.