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 AdMech's mechadendrites 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.
The most common choppa 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.
An odd variant of a Choppa.
The catch-all term for a weaponized gauntlet or a non-powered lightning claw that is sometimes filled with actual claws from animals, or knifes attached to gloves like Wolverine. No self-respecting Ork would be caught without some form of a weapon: a big knife or a set of heavy knuckles is an essential item in Ork society.
Even Grots equip themselves with carefully honed little fungus knives they use to defend themselves. In fact, many Ork brewhouses and shops won't even let Orks without some form of weapon in, so that they won't be a liability if (i.e., when) a fight breaks out.
Knuckles made their first Vidya Game appearance in the Eternal Crusade, not as if anyone remembers because of how disappointing that game was. Nevertheless, they came in various designs ranging from leather gloves with knives attached to them, to more exotic variants such as a bone gauntlet made from the claws of some beast that was killed by the Orks. Knuckles are often primitive and savage weapons even by the standards of Orks.
Big Choppas, also known as 'Uge Choppas or Big Clubs are enormously massive two-handed Ork weapons, that serve as the mark of a real connoisseur of brutality, even amongst Ork warriors. They can take many forms from great axes with jagged metal teeth to heavy clubs with plenty of spikes to make them more "killy" and give them a more aggressive look. So long as it is big enough that an Ork needs to carry it in two hands, it is counted as a Big Choppa.
Their unsubtle design and massive weight is capable of packing a considerably harder hit than regular Choppas - wielded with a overhead swing which will turn most opponents into splutchy pancakes on hit. They are are chosen more for sheer weight than subtlety and require both hands to carry and wield them. Big Choppas are usually wielded by Ork Nobs or Warbosses who can do even more damage with it using their great strength and skill.