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're plenty 'ard and plenty killy by surviving lots of battles and growing bigger and meaner by being dead killy by suicidally taking on things bigger than themselves. 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, Nobz are probably the most dangerous to encounter for the average sentient/non-sentient/inanimate object in the 40k universe. An easy equivalent of the Nob is of a Mini-Boss in a video game. They can be especially vain, as shown by their many trophies and flashy bitz. No other lower Ork is allowed as good of gear as the Nobz (to the point where if a Nob sees something he remotely 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. They weigh almost twice as much, and all of that extra weight is hard, green muscle. The only thing that can boss a Nob around that is NOT a Warboss or Warlord are Meganobz which are like the chief executor of Ork society.
While Nobz gain quite a bit of prowess in the brute force and killy department, the boost in strength comes at a bit of a cost. While there are exceptions to this, Ork Nobz have a tendency to overestimate their strength, to the point of even rushing headlong into a Titan if they believed they could take it on.
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 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.
Fun note: The new Primaris Marines, who are larger, stronger, tougher and Skubbier Marines, are rulewise the Nobz to the normal Marine's Orks. Just like Nobz, they get +1 to their W and A stats, just like Nobz. Feel free to paint your NuMarines greenfaced.
Types of Nobz
Nobz comes in all flavors and colors. However there are three official variations of Ork Nobz. Feral Ork now has their own version.
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 around the Boss. He is honored by being placed in charge of keeping the WAAAGH! Banner aloft which is the thing that comes the closest to a sacred artifact amongst Orks. The WAAAGH! Banner serves to attract the attention of the Ork Gods Gork and Mork, ensuring 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.