As said before Nodes(Hull) and Modules are the principal components to build a ship. Nodes will build the 'body' while Modules will add extra features and behaviors like maximum speed, acceleration, radar range and alot more.
Modules can only be placed over primitive Nodes (cubes) and some of them (like engines) can only be put in certain faces of the Node.
I just updated a small video showing how Modules placement works.
Like the skin on the hull.