Wood Release (Mokuton) is an advanced nature kekkei genkai that combines earth and water-based chakra natures to create wood or complete trees.
At the same time it is a kekkei genkai. These (Bloodline Limit techniques) are abilities passed down genetically in specific clans.
Some basics:
There are five basic types into which the nature of chakra can be transformed, called the five elements. They are water, fire, lightning, earth and wind. These elements allow for elemental ninjutsu to be performed. Eg: Fire Release. Wind Release etc.
In general, every person has chakra that has an affinity towards one of these types, a characteristic that seems to be genetic. (Most members of the Uchiha clan, for example, had an affinity towards the Fire nature.). This affinity makes it easier to learn how to create and control that particular nature.
Nature transformation: It is an advanced form of chakra control. It involves moulding and defining of one's chakra into one type of chakra nature, altering its properties and characteristics. Nature transformation is one of two necessary techniques for creating a technique. Basically what nature transformation means is a ninja can learn to transform his chakra into the element for he which has the affinity. So Sauke can create Fire by transforming his chakra into fire and then spit it out. A (raikage) creates lightning this way. Tobirama creates water this way.
Advanced Nature transformation: By initiating more than one basic chakra nature transformation simultaneously, one can create a completely new nature of chakra with unique properties. Doing this requires a special ability and such abilities are rarely seen outside a kekkei genkai.
So Haku can creates Ice by combining water and wind natures (Ice release). Mei can create lava by combining fire and earth natures (lava release). Hashirama can create wood by combining earth and water natures. Onoki creates dust by combining three natures - (Only one of this type in the manga sofar).
What is important to note is advanced nature transformation (combining two or more natures to create a new nature) requires a special ability and is normally a kekkei genkai ability.
And regarding kekkei genkei wikia says: Although kekkei genkai are usually genetically shared within a specific clan, sometimes it is unique to one person alone, and that even family relatives do not share it, such as the Wood Release and Shikotsumyaku (Kimimaru’s). Yet other kekkei genkai seem to appear in individuals with no known relation, such as the Lava Release, which has appeared in ninja from Kirigakure, Iwagakure, and Kumogakure, while Magnet Release, which has appeared in ninja from both Kumogakure and Sunagakure.
