Wood release is a very unique kekkei genkai limited to Hashirama Senju, the founder and 1st Hokage of Konohagakure. Users shown later to have the ability to use wood, were all shinobi with Hashiramas DNA cells. To be able to use wood one has to have great control and mastery over water and earth release elements and once he molds them together he gets wood chakra which allows him to grow trees from the ground, in shape of giant forests and such, and from his body, as branches and pillars. Wood is considered one of the most versatile elements and it has a great variety of uses.
Wood is weak against lightning because it would shred through it. It's neutral to fire and wind, although strong wind techniques could cut the branches of the trees. Finally, it's strong against water and earth techniques. It was also shown that it's great against bijuu chakra and that it has suppressing abilities. Wood is often referred to as "the source of life".