SLA is a process whereby a laser converts a liquid photopolymer into a solid plastic layer by layer. Each layer is different and a 3D model is built up on a perforated plate in the bath of photopolymer.
There is now a huge range of SLA materials with very good mechanical properties. We uses DSM Somos 14120 which closely simulates ABS thermoplastic. And we mostly uses SLA models or CNC models as Master Patterns to make silicone rubber molds for vacuum casting of Polyurethane.
http://www.dsm.com/en_US/downloads/dsms/14120eng_10.09.pdf
Selective Laser Sintering was commercially developed in the mid 1980′s in Texas, although a similar process dates to 1979. It too is a laminated manufacturing process, but uses a much higher power laser to sinter plastic powder together to form a 3D prototype. The most typical material used is Polyamide/Nylon. It is also possible to have glass filled Nylon. Historically it was considered that SLS parts were more durable than SLA, but as SLA materials improve, this gap is narrowing. SLS is also considered quite wasteful as up to 30% of all non-converted powder removed so as to refresh the powder stock. SLA has almost no material waste.
http://en.wikipedia.org/wiki/Selective_laser_sintering