SLA  is short for Stereolithography, and  SLS is short for Selective Laser Sintering, they are the additive manufacture process similar to 3D printing, both of them are applied the laser technology to make parts, SLA and SLS are widely used for quick models in complex structures like medical models, skeleton models and pipe connectors, they are widely used in  rapid prototyping and low volume production industrial much earlier than 3D printing, so they are a mature technology.
Thoutgh they are all laser technology in similar use, the products are made in different result by using different material and theories. SLA is using liquid material and cured layers by layers under the laser light while SLS is using the powder material and binding the material together by laser.