Wood is produced by esignating rees to be chopped down. Any dwarf with the wood cutting labor enabled and access to a battle axe will cut down the trees, which will turn one tree into one log, the raw form of wood.
"Timber" is the name of the ninth month of the dwarven calendar, covering late Fall.
Trees start their lives as saplings. Saplings cannot be cut down until they mature into full-grown trees, which can take several years. Saplings will randomly appear in appropriate outdoors soil to provide a slow (but steady) supply of wood. If you have discovered a cave pool or cave river, certain (muddy/muddied) areas underground will spawn tower-cap mushrooms, which can also be harvested for wood. You can also muddy soil similar to preparing it for farming and leave it unattended for a chance at growth. Fully-grown trees will impede units' movement, so be sure to clear them out of active corridors.
Besides cutting down trees and tower-caps, wood (and some wooden goods, such as barrels) is often available from the elven, dwarven and human caravans. Wood can also be purchased before embarking. Wood is quite inexpensive, costing only 3☼ per log, and you may wish to bring a large number of logs when embarking in order to jump-start your wood industry. The wagon you start the game with can also be dismantled for three tower-cap logs.
 Reasons you need wood
- To build beds
- Without beds your dwarves will get unhappy thoughts from sleeping on the ground
- To build water wheels and windmills, as well as axles
- Without wood, you cannot generate or transfer power.
- To build siege engines and ballista bolts
- These can be very effective defenses when traps fail.
- If you want obsidian short swords, they require one obsidian stone and one wood each (these swords likely consist of a thin wooden "paddle" with sharp flakes of obsidian forming sharp edges, like the Aztec macuahuitl).
- If you have access to obsidian, these can be a great source of quick weaponry early in the game, before any steel works are up to speed. Even on a tree-lite map, each weapon takes less wood to produce than a steel weapon (unless you are using magma to fuel your smelters and forges and have access to bituminous coal and lignite).
 Reasons you want wood
- It is simpler to make items from wood.
- Wood can be burnt to produce charcoal and ash, which are important ingredients in other tasks such as smelting ore, forging metal items, glass making, fertilizer for crops, and other uses.
- All metalworks (smelters, forges) and glassworks are either coal-fueled or magma-fueled. If you are planning on having any sort of serious metal or glass production, then you're going to need a lot of wood, or magma (and charcoal or coal for steel).
 Reasons you don't need much wood
- Everything other than beds, axles, windmills, water wheels, obsidian shortswords, siege engine parts, and ballista bolts can be made without the use of wood.
- If you have magma then you don't need wood for fuel. If you have coal, you don't need (as much) wood to produce charcoal for steel. If you have both, you don't need wood to produce metal or steel products.
- You can supplement your wood supply to a small degree via trading.
- If you're lucky enough to play in an area with an underground pool or underground river then you can grow your own wood supply underground with tower-caps.
Every different type of log (chestnut, ash, maple, tower-cap, etc.) is functionally identical except for their weight. The weight of a 'unit' of each type of wood is half their density; the densities for each individual type of wood is listed under the appropriate tree. Wood has a default [SOLID_DENSITY] of 500, making it about three times lighter than most stone and fifteen times lighter than iron. Feather tree wood is extremely light, with a density of 100, and glumprong wood is the heaviest, with a density of 1200. However, since average wood is relatively light to begin with, with the possible exception of wood hauling, this makes (almost?) no practical difference in the daily routine of a fortress or your dwarves.