A noble is a member of a nobility, which is a member of a kingdom's royalty. Feckin' Nobles. Due to this, nobles are the richest louts in a kingdom, having plenty of money, land, and usually soldiers under their control. Since they own most of the kingdom's resources, nobles are the most influential and powerful people in the land, normally in charge of settlements, estates, and industries. Because nobles own everything they need to survive, the commonfolk must follow their whims and decrees, for better or for worse.
Nobles who aren't completely drunk on power know that peasants need him as much as he needs them (after all, without peasants, who's gonna work for you to generate money and pay you taxes?), so they do their best in keeping the people happy, or at the very least treat them like human beings. Those who're on an eternal power trip usually don't last long when the peasants have had enough and decide to rebel en masse. Or at least you would think that it would work like that, but the fact of the matter is that heavily armored cavalry tend to crush peasant rebellions in most cases so most of them don't particularly care about how badly they treat their peasants. Rebellion is made even more difficult by the fact that often tyranical nobles in the real world would grant favors to a portion of the peasant population at the expense of all the other peasants, pitting the peasants against each other. A more modern example of this tactic would be in Colonial Africa, where Europeans would often favor one ethnic group in a region over the others, breeding resentment and making it harder for the Africans to unite against their oppressors (and unsurprisingly leading to continued strife between said groups that has sometimes culminated in genocide, like the Hutu-Tutsi conflicts in Rwanda).
Nobles in Fantasy
When the word "nobles" comes to mind in fantasy settings; the stereotype for them is the rich folk who lives it up like a pre-revolution French king while his peasants and guards do everything for him. This is only a half-truth, as while nobles can be as decadent as they wish since they can afford to do so, even noble classes partook in combat. A simple example are the Knights, who are nobles but are popular in many settings for their elite combat prowess.
Nobles throughout world history were notorious for exploiting their power to oppress the common populace, so they were usually good as enemy leader fodder in fantasy settings where the nobles continued to live a life of unparalleled luxury while the people lived in constant hardship due to him either short-changing the people on resources or taxing them to unfair levels to maintain his growing treasury. Perhaps this type of noble is no more evident in Dorf Fortress, where every living soul despises them for requesting the most luxurious and expensive goods and accommodations and will jail a worker if they fail to appease him (regardless if he's one of your most valuable, skilled workers). This type of attitude however, happens to have a particularly hilarious side-effect of them meeting "unfortunate accidents" at an unprecedented rate.
That said, the "common touch" Lawful Good noble who stands aloof from his kinsmen because of his just policies and good management is an equally-popular subject in fiction. Sadly, settings that involve strife means that a goody-two-shoes noble is highly susceptible to assassinations and chronic backstabbing attempts by rival nobles who're after his fortunes (a lot like real world history, really).
"Noble" as a character class
Dungeons & Dragons 3rd Edition included the Aristocrat as an NPC class. Like non-Adept NPC classes, it’s too weak for player use. Power of Faerun would introduce the 3 level Merchant Prince(ss) Prestige Class. Anyone with sufficiently high intelligence and Profession as a class skill (read: Anyone but Barbarian and Aristocrat) can access this at the earliest possible point, but practically a class with high skill points is required. The three required feats are all of minimal use and most of its abilities are only useful for their interaction with the subsystems in the same book, but it does gain a small bit of a casting from a limited list.
In Pathfinder, Vigilante has several class feature options that support playing it as a “noble” class. Paths of Prestige introduced the Noble Scion class, which is really just a social skill monkey chassis that’s only not unsuitable for a player character because it gives free leadership twice.
Star Wars D20 included the Noble class as a quasi Bard analog and, like all classes in the system, is a huge mess, but still one of the best non-force users in the system and one of only two classes with diplomacy as a class skill. Saga Edition's changes made the noble into a social skill monkey by default, with talent options allowing the class to function as a military officer, social butterfly, swashbuckler or more without necessarily being of nobility, rendering the name an artifact of the previous edition.