Based on materials analysis, Polyvinyl Alcohol (PVA) is a type of petroleum-based plastic that is water-soluble. It is biodegradable and recyclable, and non-toxic. It can be extruded at temperatures of 190-210 deg C. It is relatively expensive compared to materials such as PLA and ABS. Due to its ability to dissolve in water, PVA-based material must be stored carefully, away from water and moisture. For example, it can be stored in sealed bags with silica gel. Once removed from its packaging PVA can absorb moisture.  

Since PVA is water-soluble, it is used for providing support for overhanging portions of objects being 3D printed. The 3D printer has to be capable of extruding two materials. A PVA-based structure is 3D printed before the overhang portions. The overhang portions are 3D printed, with support from the PVA-based structure. The final 3D printed object is put in water to let the PVA-based support structure dissolve. After the PVA-based support structure dissolves, what remains is the 3D printed object with overhangs.

3D PRINTERFILAMENT COMPARISON GUIDE.

Free Online 3D Printing Resource.