Log

"I'd estimate that about 95 percent of my Minecraft worlds begin with me collecting a single block of Oak Wood. It's the mother-block - the block from which everything else in the game develops. Without collecting wood, it's almost impossible to play Minecraft. Seriously, try playing Minecraft without wood. It'll be about as much fun as playing blindfolded, or with me screaming "USE WOOD YOU FOOL!" into your ear the entire time."

- Duncan Geere

Wood, or log (from the name ID), is a naturally occurring block found in trees, primarily used to create wood planks. It comes in six species: oak, birch, spruce, jungle, dark oak, and acacia.

Bark is a variant that has the log texture on six sides, and comes in the same species as wood.

Stripped log is a variant obtained by right-clicking a log with any axe.

Obtaining
Wood can be broken by hand, but using an axe will speed up the process.

Natural generation
Oak wood blocks generate as part of houses in plains villages, acacia wood blocks in case of a savanna village and spruce wood blocks in case of a taiga village. Spruce wood blocks generate as supporting beams of witch huts. Oak and dark oak wood blocks generate as part of woodland mansions.

Oak, Spruce and Dark Oak Stripped Log generate as pole in shipwreck.

Trees
Wood blocks of all six types generate naturally as part of trees. Trees can also be grown using saplings.

Stripped logs
an axe on wood will turn it into a stripped log.

Further crafting
The following table presents the amount of wood needed to produce an even multiple of a given item with no waste left over, and the quantity produced:

Fuel
Wood logs and bark blocks can be used as a fuel in furnaces, smelting 1.5 items per block. It is far more efficient to craft each wood block into four wood planks: each plank will burn as long as the log would have, for a total of 6 items.

Smelting wood into charcoal is slightly better: each charcoal will smelt 8 items, but the charcoal itself needs to be smelted. The net items smelted therefore is 7. Also, the increased need to load and unload items will usually result in some loss of smelting time.

Data values
Oak, spruce, birch, and jungle wood and bark blocks have the ID name  and are further defined by their block data or block state. Acacia and dark oak wood and bark blocks have the ID name  and are further defined by their block data or block state.

Note that when a wood block is placed, it changes its facing parameters, placing in the direction relative to the block it is placed on.

Block data
The block data for wood blocks consists of two facing bits (high-order) and two wood type bits (low-order).

Block 17 

Block 162 

Block state
Block 17 

Block 162 

Trivia

 * Dark oak and acacia logs share a new block ID separate from the old logs, called log2, because all the other data values on the previous log block were occupied by rotated variants (red sandstone slabs suffered a similar fate). There are still 8 remaining data values on the block that cannot be obtained. The next two trees, if more are added, will likely use these values.
 * Rotated logs in the inventory will be placed like normal logs, even if the item is an all bark block.