The StereoLithography (SLA) process, in fact any of the RP processes, begins by exporting your 3D CAD design from your CAD package in STL format. 

The 3D design is then “sliced” in software and then built, layer by layer (or slice by slice) in the SLA machine. The laser that traces each layer emits UV energy and the material in the build chamber is a UV curable epoxy resin. The laser traces each layer and bonds successive layers to the previous layer.

The material used to build the part is a UV curable epoxy resin. The material is like a 2 part epoxy where the second part is energy from a UV light and in the case of SLA, the energy comes from a precision guided UV laser.

After the entire part is built, the part is removed from the machine and cleaned (with a soft bristle brush) in a material such as IPA to remove any excess resin. The part is then cured in a UV oven, basically a small tanning bed, and is ready for any secondary operation that is needed.

A video explaining the SLA process can be watched here.