Among the ways to deposit material in a layer-by-layer approach, one is to use jetting technology. Jetting material in this manner is akin to inkjet printing. But, instead of ink as the material, the material is usually a type of photopolymer or wax. (Other vendors use inkjet approaches and use other materials, including binders.) A light source, typically UV light, cures the material.
PolyJet 3D printing specifically refers to those material jetting printers made by Stratasys. Other vendors that offer an inkjet type of 3D printing use other names, including MultiJet 3D printing.
The major benefit of this type of 3D printing is that it produces smooth, accurate parts, delivering good part finish that needs little post-processing. With the Stratasys systems, layer resolution is good and part accuracy is 0.014 mm. This technology also has the ability to produce thin walls.
Most jetting approaches to 3D printing use multiple depositing heads to lay the material, which are deposited as droplets.