A Nob (an old word meaning a person of wealth or high social position, not just a big willie) is an Ork noble that lies between a Warboss and an ordinary Ork boy. This means that they've grown plenty 'ard by surviving lots of battles and growing bigger by killing anything that they've come across. Nobs either tend to stick among themselves and form squads or look over other smaller Orks and make sure they're not mukkin' about.
Fluff wise, running into a Nob would be a highly dangerous encounter in the 40k universe, being the equivalent of a video game mini-boss. They can also be quite vain, as shown by their many trophies and flashy bitz. No other lower Ork is allowed to have wargear equaling that of the Nobz (to the point where if a Nob sees something he is interested in, it is confiscated immediately), with the exception of the Warboss himself. The average Ork Nob stands a good head taller and two shoulders wider than the average Ork Boy and weighs almost twice as much, with all that extra weight being hard, green muscle. The only thing that can boss a Nob around that is even larger Orks such as Meganobz or the Warboss himself.
While Nobz as a rule are not necessarily known for their intelligence, it must be stated that they are fighters and survivors above all else. This experience does often lend a certain cunning that can make them even more dangerous. If Nobz have a weakness, it is that they have a tendency to be highly overconfident and to overestimate their own strength and ability.
On the tabletop Nobs are somewhat costly, and rather situational without a certain use in mind: Nob Bikers may find a place in certain lists, and Meganobz can krump just about anything, while a regular Nob unit equipped entirely with Big Choppas, Chain Choppas and Big Chain Choppas which can be moderately useful against multi-wound infantry or similar units. A "shooty" mob build is also possible, but of debatable utility compared to simply taking more Boyz.
Types of Nobz
Nobz on foot are most commonly seen leading bands of Ork Boyz into battle. However, Nobz will occasionally form into mobs of their own, where they function as elite Ork infantry. On the tabletop in 8th/9th edition, Nobz are about twice as expensive as a regular Boy. In exchange they have S5, 2 wounds, an extra attack, a 4+ save, and Ld7. Compared to any other unit in the Codex, Nobz have the greatest range of options when it comes to their equipment. Each Nob comes with a Slugga and a Choppa by default. Either of these weapons may be replaced by a Big Choppa, Killsaw, Power Klaw, Power Stabba, a 2nd Choppa, or even a 2nd Slugga. Alternatively, the Nob may replace all of his weapons with either a Combi-Rokkit or a Combi-Skorcha. It is also possible to enhance the mob with additional upgrades such as Cybork bodies and ammo runts. While this wide range of options can become expensive if you go overboard on them, the advantage is that each Nob can be flexibly equipped as desired. The current Nobz plastic kit is quite excellent in this regard.
A common sight in Evil Sunz and Speed Freeks warbands, Biker Nobz are, as the name implies, a group of Nobz fighting as a unit where each member rides a heavily "kustomized" Warbike. Such a unit is a terrifying foe to face, as the speed and firepower of the Warbikes combined with their riders' prowess at close quarters combat allows the Biker Nobz to crush almost anything standing in their way.
The richest and most battle-hardened Nobz can become Meganobz by paying a Mek to build them a suit of Mega Armour. Piston-driven and covered in enough protective plating to turn its wearer into a walking tank, this armor is incredibly expensive, yet it serves as the ultimate Greenskin status symbol.
WAAAGH! Banner Bearer
The banner bearer of a WAAAGH! is often the Warboss' Gak'ena (an Ork term meaning at the same time "best friend" and "worst foe"), and is the strongest, largest and meanest of the Nobz aside from the Boss himself. He is honored by being placed in charge of the warband's WAAAGH! Banner, which is the closest thing to a sacred artifact amongst Orks. The WAAAGH! Banner serves to attract the attention of the Ork Gods Gork and Mork, ensuring that their blessing is always with the Orks. It also doubles as a more mundane means to transmit the Warboss' "orders", which means that it is brandished in the general direction the Boss wants his troops to charge.