The yew, aka Taxus spp, is a tree/shrub that grows predominatly in wet, temperate climates.
The most noticable thing about the yew is that it is one of the few gymnosperms that has berries, or at least what appears to be berries. Much like the alder, which has fruit that appears to be cones, the yew has cones that appear to be berries. They are little fleshy growths that cover most of the seed.
Other then the red berry like cones, Yews can be recognized by their dark green needles and their size, which is usually 2 to 5 meters, and with multiple stems.
All parts of the yew are said to be extremly posionous, and as little as 2 or 3 needles is enough to kill a child. I myself have eaten that much, but being about 70 kilos, I was unharmed. The specific poison in yew is a glycoside that blocks the calcium channels in muscle tissue. In heart tissue, this means the heart doesn't beat. As you would guess, it is a very hard poison to deal with, and there is no antidote.
In addition, one species of yew, the Pacific Yew, produces a chemical called Taxol, which can be used to treat breast cancer. However, this takes about 5 tons of yew bark to make a dose of it.
The greatest traditional use of the wood of the yew was as the stock of a bow due to its strength and flexibility of the wood. In addition, a single stem of a yew tree was about the right size and width for a bow stock. Since the era of polymers etc, and the decline of the bow and arrow in warfare, this use has probably been greatly curtailed.
There is various mythological associations with the yew tree, probably due to the fact that it is poisonous and had bright red berries.