3D printing is a process of making three dimensional solid objects from a digital design file.
A 3D printed objects is achieved by using an additive process (rather than a subtractive process). The object is created by laying down sccessive layers of material until the object is created. Each layer can be seen as a thinly sliced horizontal cross-section of the eventual object.
A milling machine which cuts out or hollows out a piece of metal, plastic or wood is an example of a subtractive manufacturing process which is essentially the opposite of 3D printing.
3D printing allows complex shapes to be created with less wastage of material than traditional manufacturing methods.