Mechanical
This is the type of process you will be most familiar with. It involves the use of cutting tools (the teeth on a saw or a file, a lathe cutting tool or drill bit) or abrasive particles bonded as a solid (disc sander, grinding wheel) or fluid (sand-blasting, ultrasonic machining).

Electrical
The most common method of electrical machining is spark erosion. It works by generating sparks at voltages from 20-500 V between an electrode and the work piece (which must be a metal). The work piece must be contained in a semi-conducting liquid such as paraffin. It is used to machine hardened tool steels and to create a grainy texture on injection moulded products.

Thermal
Thermal techniques remove material by melting or vaporising it. Oxyacetylene cutting uses gas to melt through steel plate from 6 - 150 mm thick. Lasers can be used on both ferrous non-ferrous materials such as textiles. There is less distortion compared to oxyacetylene cutting but it is limited to thicknesses of about 20 mm.
Hot wire cutting is used to cut materials such as polystyrene. A low voltage is passed through a wire causing it to heat up to the melting point.