Lumber can be used in any ways of the following:
Early on in the game lumber is very important, and, when one is under protection, slightly hard to obtain. Fields use much lumber, as does the Science Center and several other buildings. However, later on in the game lumber usually ceases to be a troublesome resource.
Where to get lumber?Edit
Unlike food, there is not so much lumber in Anthropus Camps so most players use Lumbermill and Forest production way, but can also be obtained in mass by raiding rivaling cities. You can also join an alliance and have them send it to you, or buy it through trade.