Subculture is an informal game term. Subcultures are divisions within cultures, usually based on race or an Unloaded Keyword. In particular, Isengard and Raider cultures are each split up into subcultures, each of them with their own subcultural enforcement.
Isengard is split cleanly into three or four subcultures: Uruk-hai, Isenorcs, Warg-riders, and (arguably) Isengard Men. Each subculture is largely siloed into cards that work with that subculture and only that subculture. Saruman, Keeper of Isengard (3R68) and Savagery to Match Their Numbers (1R139) are only useful in an Uruk deck. Warg mounts can only be played on warg-riders, which are the only minions that care very much about Saruman, Master of Foul Folk (5R56) 's healing. Only Isenorcs can be stacked on cards like Gnawing, Biting, Hacking, Burning (6U63) or Scaffolding (6U73) . Isengard men don't work with any of these cards, but their own cards, like Rohirrim Traitor (6C72) , Unferth, Grima's Bodyguard (4C178) , Leechcraft (4R157) , and Desertion (6U61) , are so weak that they're barely played at all. There are cards that support the whole culture, like Saruman's Power (1U136) or the Isengard weather cards, but there's little reason to ever play both Uruks and Isenorcs in the same deck.
Raider culture is similarly divided into the Oliphaunt-riding archer Southrons, the armored Easterlings, and the sea-faring Corsairs. In this case, each subculture has their own special mechanic: Southrons generate Twilight later in the turn with Ambush and have their own exclusive Mûmak (5C73) mounts. Easterlings generate and spot burdens for benefits, particularly in the Beasterlings deck. Corsairs have effects when they come into play and pile up Culture Tokens on their support area ship possessions like Corsair War Galley (8U59) and Ships of Great Draught (8R65) . Raider decks aren't quite as tightly bound to their subcultures: Southrons have expensive events that are meant to be funded with Ambush like Red Wrath (7U157) and Fierce in Despair (7R148) , but any Raider deck can use them if they have the Twilight. Ships of Great Draught (8R65) will have many more tokens if you use minions like Corsair Pillager , but any Raider deck can just generate tokens using the card itself. Even so, it's rare to see a deck straddle two or more of these subcultures; it's more common that you see some of the best cards from another subculture in a deck primarily focusing on one of the three subcultures.
While other cultures have subcultures, most of them have a large critical mass of cards that interact with all cards from that culture, so the subcultures never feel very restrictive. Those subcultures do exist, though.
Shire culture often has a divide between cards that apply to Ring-bound hobbits (Frodo, Sam, and later Bilbo) and unbound hobbits (Merry, Pippin, and later the other minor-character hobbits). This divide is keenly felt in some formats, especially when Legolas, Dauntless Hunter (4R73) is a meta-defining card, but less important in others. Every Shire deck splits the difference a little. It's impossible to have an unbound Ring-bearer, and there are so few Ring-bound hobbits that it's hard to build a deck that focuses entirely on them.
Ring-bound Rangers (also known as "roaming Rangers") are Faramir's scouting company, and they focus on increasing the site number of minions, then punishing roaming minions in various ways. (This subculture turns up again in The Two Towers, this subculture turns up again years later in The Hunters, albeit without using the ring-bound keyword.) Cards that specifically refer to ring-bound Gondor characters or affect minions site numbers or target roaming minions are generally part of this subculture. This is not a popular subculture except in Expanded, where the Madril, Defender of Osgiliath (15R64) / Ithilien Blade (15C62) combo is fairly infamous. It's also not strongly enforced, because many unrelated cards refer to Faramir or rangers or Gondor companions in general.
Valiant is an odd Rohan subculture. It doesn't have a theme, just some relatively more-expensive companions that are spotted by some stronger cards, like Simbelmynë (4R289) , Let Us Be Swift (5C85) , and Éomer, Valiant Warchief (7P365) . The only subcultural enforcement is that you need to spot a relatively large number of Valiant men for some of these cards.
Ent decks generally use large, powerful companions that get a Twilight discount from spotting other ents, like Ent Horde (6R28) . These decks are generally so focused on playing ents and running down the site path that they don't run many other cards. That said, these decks almost always include Merry and Pippin and occasionally find room for Gandalf or Radagast, The Brown (9R+26) .
Orc Lurker minions tend to have abilities that buff or recur each other, or benefit from being at Underground sites. The Lurker keyword itself serves as soft cultural enforcement: any minion that doesn't have Lurker is liable to die first. This is a very soft subculture, though, since there's nothing stopping you from playing generally-useful cards like Orkish Scout (17R84) or Pitiless Orc (12U99) in a Lurker Orc deck, and several of the Lurkers' key cards -- especially Goblin Hordes (11R123) and Cave Troll of Moria, Savage Menace (12R85) -- appear frequently in other Orc decks.
This list isn't exhaustive. There are many other possible subcultures that could be listed here, including Gondor knights and their fortifications, Sauron trackers, Sauron roaming orcs, or Sauron besiegers. These are just some examples.