Of all the different types of material used in FDM/FFF printing, the two most popular materials are PLA and ABS. Both of these materials are types of thermoplastics. Thermoplastics are plastics that are hard and solid when cooled and moldable and soft when heated. Thermoplastics are common in society and make up most of the plastic objects that we are familiar with, but only a select few types are suitable for 3D printing (image courtesy of 3dhubs.com).
In order for a thermoplastic to be viable as a 3D printed material, it must fulfill 3 criteria:
1) The thermoplastic must be able to be extruded into plastic filament. In other words, it must be able to take the form of the 3D printing filament on the spools that you are probably familiar with.
2) It must be able to bind to the build plate and to itself in the 3D printing process while giving a smooth and accurate result.
3) It must be appropriate as a material choice for the end use application of the 3D printed part. The properties of the material should fit with its intended application (whether it needs to be strong, durable, shiny, etc.).
PLA and ABS have similar qualities and are suitable for a number of different applications, but they also differ in methods of operation and chemical makeup.
PLA or Polylactic Acid is created from processing a number of plant products such as corn, potatoes, or sugar-beets. PLA is biodegradable and can be composted at commercial compost facilities (it will not biodegrade itself in your backyard). It is considered to be much more earth-friendly and is stronger and more rigid than ABS (image courtesy of NatureWorks). PLA isn't as popular for objects that need to withstand heat for an extended period of time as it has a lower melting point than ABS. Both PLA and ABS can be sanded and machined to reach the desired outcome of the user.