Styrofoam, a brand name for polystyrene, is a very flexible crafting and building material that you can buy in blocks or sheets. To create items thicker or larger than the sheets you can glue several together with styrofoam-specific glues. Cutting styrofoam can be a messy business, and while special tools aren't required they can improve your precision and reduce waste and mess.
Serrated knives cut through styrofoam easily. You can use steak knives to cut small pointed divots in your styrofoam. You can even use a steak knife to cut through some thinner styrofoam sheets, however the curve in most steak knives often causes compression marks on the styrofoam. Bread knives are better for cutting styrofoam having a flat, serrated edge and being longer. When cutting the styrofoam with a bread knife, try to keep the knife perpendicular to the cut and move forward steadily. The drawback to cutting styrofoam with a serrated blade is that it is messy and small, statically charged bits of foam will litter your work area.
Electric knives used to cut bread or carve turkeys work well on styrofoam sheets. It's important to have your design planned out and marked on the sheet of styrofoam before beginning your cut. Keeping the electric knife moving will help give your cut a smooth edge. Both powered and unpowered serrated knives leave many small pieces of debris after cutting. Running a vacuum nearby while cutting will capture some of the polystyrene debris. Some electrical knives have thinner blades which will reduce the drag on the blade and make cutting curves easier.
Hot-wire tools use a reduced current, usually by incorporating a transformer in the set-up, to heat a wire held between two metal arms. The metal arms of the hot-wire cutter attach to a plastic or wooden nonconducting handle. The heated wire will melt its way through the styrofoam leaving a clean edge without producing debris. The hot-wire cutting tools are good for large cuts but are limited by the distance between the blade and the handle. To use these on large sheets of styrofoam, you may need to sacrifice the pieces around the pattern by cutting away the excess as you go.
Shaped Heated Tools
Many small, heated tools work on the same principle as a hot-wire styrofoam-cutting tool, and you use them to gouge, or melt away, portions of styrofoam, rather than cut through large sheets. Some tools that look like a screwdriver melt holes in the rigid foam. Others have curved filaments that can melt curves and dips into the polystyrene piece. You can also use wood-burning tools, which usually come with different shaped tips. But, use these with a light dimming switch or a regulator to reduce the temperature of the tool. You want to melt the rigid foam not burn it.