'Bogwood' is a term used to describe the wood sold for, and suitable for, aquarium use.
It is called bogwood because it is most commonly dug up from peat bogs, usually as a secondary product during the collection of peat for gardening purposes.
Most forms of recently dead wood quickly rot, producing bacterial and fungal blooms and releasing toxins in the restricted volume of an aquarium, so are entirely unsuitable. Bogwood, on the other hand, has spent hundreds of years becoming waterlogged and semi-preserved by organically rich surroundings. Because of this bogwood has two main qualities; firstly, it is safe for use in the aquarium and will not rot in a harmful way, secondly, it will sink (usually!) as it is already waterlogged.
Different forms of bogwood are available for the aquarium and may come under different names such as 'mopani wood' that has been pre-cleaned and sandblasted for a smooth appearance, or 'Jati wood'. Bogwood in the form of 'twisted roots' is also available and can be a very attractive addition to an aquarium.
Bogwood often has unique twists, knots and shapes due to many years of subtle alteration, which make them ideal items of interest in the aquarium.
Bogwood is more than just an item of decoration, some sucker-mouth catfish use the organisms that grow on wood to help them break down and digest food. In many cases in aquariums without bogwood, sucker-mouth catfish, especially those from the ancistrus family (bristlenoses), have mysteriously died.
Bogwood does have its disadvantages, depending upon your point of view, the most common of which is the discoloration of water it may cause. Bogwood is full of a substance called 'tannin' which is an organic pigment collected from the breakdown of organic material (leaf litter etc...) When the bogwood is placed in water, these tannins are released, giving the water a yellow-brown tea-like appearance. This is not harmful to the fish and some may even enjoy it, as the discoloured water more accurately represents the environment of many fish. A few species, normally soft-water species, may even be affected in a positive way from the tannins and can be encouraged to spawn.
If you like your water to appear crystal clear, the use of activated carbon or similar chemical medias can be used to effectively remove the tannins. Many sources advise soaking the wood before use to remove tannins although because the amount of tannin in each piece of wood is highly variable, you would need to soak a piece of wood from anywhere between a week to a year or more!
Closely linked to the tannins are humic acids, which are also produced by the bogwood, these humic acids can counter some of the natural minerals in the aquarium, and produce a lower pH and hardness level. This is not normally a problem providing regular water changes are carried out and/or the water has a reasonable hardness level to act as a buffer.