These types of cutting tools use high temperatures, and/or pressures to cut through the material.
An Oxy Acetylene torch ignites a mixture of oxygen and acetylene, to produce a flame that burns between 5800°F to 6300°F. The resulting flame is used to melt a cut through various metals.
A plasma torch uses a mixture of gas(es) and and electrical arc to generate a plasma, which is hot enough to melt through the material to be cut.
A laser is used to melt through the material.This method is typically only used in industrial applications, so it's not likely you'll use it at home.
A water Jet cutter uses a very high pressure jet of water; or water abrasive mixture, to cut through materials. Like lasers, it mostly use in industrial applications. Water Jet is more commonly used for cutting plate materials, but could possibly be used for cutting tube stock as well.
These types of cutting tools "grind" their way through materials, by removing a small bit of material at a time.
A hand grinder uses an abrasive disc (or wheel) to cut through a material. This method is often slow and tedious, and will likely require multiple discs depending on the material being cut.
A chop saw works similarly to a hand grinder, except that the disc is often larger, and is held an a consistent angle to the material being cut. Since it's not held in the operators hand, it provides a more accurate cut than a hand grinder.
Like most materials, metal can also be cut using toothed saw blades.
Similar to a chop saw, except instead of a grinding wheel, the cold saw uses a blade with teeth to cut the material. The blade spins at a lower speed than a chop saw, and uses a coolant to keep the part and blade cool while cutting.
A band saw uses a long band with teeth in it, which is rotated around a set of rollers to cut through the material.