3D printing, or additive manufacturing, builds three-dimensional objects layer by layer from a digital design (CAD model) by adding or bonding material, unlike traditional subtractive methods that cut away from a block. It uses various processes (like melting plastic filament or curing resin with light) to fuse materials (plastics, metals, powders) into complex shapes, offering high geometric freedom, customization, and less material waste, ideal for rapid prototyping, specialized parts, and complex geometries.