Not all 3D printers use the same technology. There are several ways to print and all those available are additive, differing mainly in the way layers are build to create the final object.
Some methods use melting or softening material to produce the layers. Selective Laser Sintering (SLS) and Fused Deposition Modeling (FDM) are the most common technologies using this way of 3D printing. Another method is curing a photo-reactive resin with a UV laser or another similar power source one layer at a time. The most common technology using this method is called Stereolithography (SLA)