Bogwood.
This is the most common type and is very popular and widely available. It will release tannins in the water initially; this can be reduced by boiling in water for a min of 20mins and soaking overnight before placing in the aquarium.
This wood is heavy enough to sink without any help.
The tannins will acidify the water, slightly, so keep an eye on the pH levels, mostly at the beginning.

Mopani wood.
Mopani wood is one of the hardest, densest woods in nature so it will sink like a stone un-aided. Like bogwood it will release tannins in the water initially; this can be reduced by boiling in water for a min of 20mins and soaking overnight before placing in the aquarium.
It is naturally very gnarly and it has dark and pale streaks running through it. Compared to bog wood it can be expensive and is often sold by weight rather than by piece.
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Driftwood. Wood that is floating or that has been washed ashore.
This is very natural and safe to use, however driftwood for the seashore should not be used in freshwater tanks due to the amount of salt contained in it. This wood needs to be soaked for a few weeks to become waterlogged so that it sinks.

Mangrove Root.
This is a very hard wood and looks similar to bogwood when wet. It is very good for decorations and releases very few tannins. Only requires the initial boiling.

Green wood. Not recommended.
Live wood or recently chopped wood is best avoided. It will still contain sugars and starches and as these start to breakdown, the free oxygen in the water will be reduced.
It may also contain toxins (which are the trees natural defence) and/or parasites.

Fake Wood. The most expensive and limited range nevertheless worth a mention.
Totally safe for all aquatics and plants, however remember to rinse before use to get rid of any dust etc that may be on it.

I hope some of you find this usefull

This post has been edited by danio2004: Aug 30 2004, 05:58 PM