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 when we talk about 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).
Stereolithography (SLA) is a form of additive manufacturing technology used for creating models, prototypes, patterns, and production parts in a layer by layer fashion using photopolymerization, a process in which light causes chains of molecules to link together, forming polymers.The material is Resin and ABS.
Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser as the power source to sinter powdered material (typically metal), aiming the laser automatically at points on space defined by a 3D model, binding the material together to create a solid structure.The material is Nylon and PA+GF.
Other differences between SLA and SLS.
The partial contrast parameters of the two 3D printing techniques are shown in the following figure: