Choppa
The Choppa is one of the most innovative inventions the Orks have made. From some humie git's point of view, it's merely a piece of metal attached to a stick but to an Ork, it's a tool so universal that the servo arms of the Adeptus Mechanicus would seem like toys. In-fact the tech-priests of the Mechanicus overload their CPUs trying to figure out how the crude Orks crafted such a tool.
Other Choppas
The most common choppa, as seen above, is a piece of salvaged metal sharpened on one end and attached to a stick on the other. Some gits use other things lying around that are called "choppas" for convenience. Whatever an ork has lying around that can be used in a vaguely chopping motion, and has no functioning higher technology, is, for all intents and purposes, considered a potential choppa. An ork biker mob might use chains or a spare exhaust pipe, an ork who worked with a mek or dok might have a hammer, broken tablesaw, or a gurney with discarded surgical tool hammered through it. Particularly wealthy orks might have an ork-toothed chainsword or a choppa dipped in yellow paint and with teef tied to it. A desperate ork might use a severed limb or a handy grot in times of desperation, and get a free snack out of it if comes through on the other side.
In other words, if it ain't Shooty and if it ain't Glowy, then it's a choppa!