The gas axe is a common name for the longer oxyacetylene cutting torch.
The axe uses a steady flame of compressed oxygen and acetylene to burn through materials.
It is a highly mobile tool, only requiring two canisters of compressed gas (one for each gas) and a mixing nozzle. Because of the relatively small weight and ease of use, it is very popular for cutting steel "in the field" - you find a problem, inspect it, fit the correct nozzle, light it up and cut away. A few heartbeats later you have either irrevocably damaged a fine piece of equipment, or actually cut what you wanted to cut. The smell of burnt steel should be satisfying.
By varying the amount of oxygen and acetylene added to the mix, different temperatures can be achieved (low temperature lead to less efficiency and more waste gas though). At an optimal mix the axe will burn with a short bright blue/white flame. At less efficiency, with too little oxygen being added, the torch will burn with a long orange flame. This will produce lots of free floating carbon, but the inefficient flame is often (incorrectly) used for removing moisture from an area that is later to be welded.
The gas burns at a temperature above the melting point of steel, making it extremely handy for cutting most materials. The precision of the cut produced is dependent on the stability of the flame - a hand torch will leave a ugly scarred surface, while a automatic cutting robot can achieve a fairly decent surface. The surface produced will almost never be good enough for mechanical components, so for those other cutting methods are used.
The gas axe is not very efficient at joining materials because of the gas streams tendency to blow away the molten steel, but small joints can be handled. This is often considered to be an emergency backup, and only for joints that are not important (load-bearing). Welding should be used for anything requiring proper strength. Welding equipment can be as portable as the gas axe, but requires a power cable to be laid out to the site.
As it is completely self sufficient with respect to drive gas, it can be used in inert atmospheres - and even underwater. You only need a spark to light it, and that can be built into the mixing head.
The gas axe is slowly being banned from use subsea with divers though. This is because of a few accidents where unconsumed gas has built up inside enclosed spaces and then later ignited, creating a subsea shock wave that could potentially kill the diver. Even when working in airfilled enclosed spaces care should be taken to ventilate the gas - the shock wave is probably non-lethal in air, but I don't bet my life on "probably".