A 3D printer is a machine that creates prototypes and parts out of plastic or other materials using an additive manufacturing process.
Additive manufacturing produces objects by depositing a series of layers from the bottom up to form the final object. This is exactly the opposite of what happens in traditional subtractive production processes (e.g. cutters) which work by removing material from a starting block to obtain the desired final object.
Starting with computer design (CAD), which defines a toolpath, the 3D printer extrudes the melted thermoplastic filament and deposits it in layers to build the part from the bottom up. This makes even very complex parts easy to produce.