Filament is the "ink" for 3D printers, it comes in long strands wound on to a reel usually in 1.75mm or 3mm diameter, your printer should specify what diameter you should use. There are many types of filament (including wood), but the two main types are PLA and ABS. PLA (PolyLactic Acid) is a biodegradable plastic, and is the most common type of filament used for home and hobby printers. As it is made from plants, its considered to be more ecologically friendly than other filament types, smells less when printing, but it is more brittle, has a higher surface hardness and it is more prone to break when bent. The next most common type of filament is ABS. ABS (Acrylonitrile-Butadiene Styrene) is an oil-based plastic. It is a tough material that can be used to create robust plastic objects for everyday use, but smells more when printing. ABS also requires a higher print temperature, and a heated print bed (the platform the printer prints on to). The best choice to start with is PLA, but if your looking for a more robust construction and your printer has a heated print bed, try ABS.