Firewood

Firewood is a crafting material that used in many recipes. It can be directly used as a fuel in bonfire and brazier.

Firewood can also burned in a bonfire or brazier to make charcoal.

Source
Firewood can be obtained from logs. Each log will usually drop 3~4 firewood.

Recipes
It is also used in the following recipes:

Carpentry

Logging

Blacksmith

Tinkering

Mining