A Fabricator was a manufacturing device, developed by the Takarans, given the correct specifications and raw materials, it was capable of producing just about any component you might need.
Fabricators use digital designs, to accurately created material objects. They use multiple heads, and can make complex parts utilizing many different raw materials. They can even combine raw materials to make other materials. However, they cannot rearrange atoms and such to make whatever they want out of inert mass. If you want iron, you have to feed it iron. If you want plastic you have to feed it plastic, and so on.
The parts that the fabricators make, although sometimes quite complex (e.g. complex circuit board.) still have to be assembled by either a worker or another assembly machine.
The technology was developed by the Takarans, who used it extensivily on all of their military starships, in order to make them self-sufficient, allowing them to create replacement parts while on long patrol, when they are away from support.
Since then, fabricators has been extensively used on Aurora to keep her in battle ready state. For example, for the Jump Drive emitters, which are about the size of a large refrigerator, were all hand assembled using internal parts and external housings that had been created using both the large (for the housings and some internal frameworks) and small (internal parts) fabricators.