Filament is the plastic that you feed your 3D printer to create unique designs and prototypes. Each 3D printer is different in the diameter (1.75mm or 3mm, respectively) and type (PLA or ABS) that its extruder uses.
What's the difference? In short, ABS is petroleum-based, tough and impact resistant, as well as melting at higher temperatures (225-250C). PLA filament is bio-based and is generally made with corn, has a lower melting point (190-240C), and is slightly more brittle.