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 by which light causes chains of molecules to link together, forming polymers.
3D printers can use a laser or projector to project an image of light onto a liquid (resin), which will in turn harden the liquid to form a layer. By doing this in layers, you can form an object. Advantages of this technology compared to FDM are the higher resolution and faster layer times.