When people think about 3D printing, they usually visualize consumer or hobbyist FDM
machines like Makerbot
pictured here. FDM stands for "Fused Deposition Modeling", and that's what it is: molten thermoplastic is deposited on a bed to build a model layer by layer. Hundreds, if not thousands, of printer makes and models use FDM. You can get a variety of materials to deposit, including such unlikely ones as wood or carbon fibre.
Unfortunately, FDM is not really suitable for precision mechanical prototyping. The models made by FDM printers are anisotropic
, meaning their strength varies with orientation. This method is also not very precise, and, most frustratingly, the imprecisions are somewhat random.