Stereo lithography, or SLA for short, is one of the most commonly used 3D printing technologies used today. The process builds 3D parts from a photopolymer liquid resin which is hardened by a UV laser. The laser traces out the profile of each slice of the part and gradually builds a part layer by layer. After the laser has completed a slice, the build table lowers into the resin and the process starts all over again for the next layer.
Fuse Deposition Modeling or FDM is a process where the 3D part is built by extruding material (usually plastic,ABS/PLA) through a nozzle onto a build surface. The part is built up layer by layer and support material is used where ever there is an over hang.
SLS 3D Printer
Selective Laser Sintering is similar to SLA except a powder is cured instead of a resin. The advantage of SLS is that much tougher materials can be used such as glass filled nylon. The resolution of SLS printers is not as good as SLA so the surface finish is no where near as smooth as SLA but as you can used tougher materials it is great for fit and function testing and also for rapid manufacturing for non visible parts.
The parts are built by melting powder plastic with a CO2 Laser which traces around the profile of the part for each slice.
SLM 3D Printer
Selective Laser Melting is similar to SLS however it melts fine grains of metal together using a very high powered laser. It can produce metal parts in stainless steel, titanium, gold and some other metals. Parts produced can be up to 99% dense and have similar strength to machined or case parts.
There are others:
DMLS:Direct metal laser sintering
FFF: Fused Filament Fabrication
MEM:Melted and Extrusion Modeling
LOM: Laminated Object Manufacturing
EBM: Electron beam melting
SHS: Selective heat sintering