They appear as wooden blocks in some levels of the game. When they are on fire, they gradually blacken and fall apart in ashes.
It can be destroyed like bricks, except wood can be set on fire with Molotov or with gunpowder barrels. When on fire, it will burn for some seconds and when the fire is done, it will be gray and in a while will turn to dust or ash. When wood is on fire and has contact with another wood block, the other wood will catch on fire too, causing the fire to spread. In this way entire wooden buildings can be destroyed in one shot.