A shipwright is responsible for the building and the repair of the ship. Most pirates needed them for repair and general maintenance of their ships. Other ships often have them as well.

In combat Shipwrights are known to use obscure weaponry that they also use for their daily job. These include:

Hammers
Chisel
Saw
Ropes

When working a Shipwright must be able to work with certain materials. These include:

Wood
Steel

Through use of items as well as technique shipwrights are able to enhance their ship.
In regards to fighting a shipwright will only be able to master and utilize one style. They will only be able to use one weapon to its prime effectiveness depending on which NPC shipwright they wish to learn from.
There are Type1, Type2 and Type3 Shipwrights. You can tell which type a shipwright NPC is by looking at their name. Once you learn from one type you can only learn from that type.