Wood is one of the most common, and most widely used, resources in Ark. It is harvested from various trees and can be effectively harvested with various tools and a few Dinos. It is also possible to harvest this with your bare hands, though it is not very effective and does cause injury.
Whether for fire, wood structures or stone structures, you'll be using a lot of wood. It is a conerstone resource in Ark, and the more ambitious your projects, the more you will need.
Wood, like Thatch, can be harvested by hand. However, either with a tool or Dino power is the best method for collecting this resource. The Pick, whether metal or stone, does work but will also produce much more Thatch than Wood.
To harvest Wood well you'll want the Stone or Metal Hatchet. The Metal Hatchet is far superior to the Stone one, but is a bit more work to produce and maintain.
For any project that involves large amounts of wood, nothing beats Dino Power! So far I have found the Mammoth to be the best Dino for the job. With it you can harvest hundreds of wood very quickly. You'll also produce some Thatch and a ton of berries and seeds if there are bushes around. Depending on your Dino's weight capacity, you could easily harvest up to and beyond 700 wood in a single run into the forest.