WARMACHINE
Warmachine is based, along with Hordes, in the Iron Kingdoms, as featured in the 3e D&D Iron Kingdoms books. There are noticeable differences between the RP and wargame versions of the setting.
Gameplay
Armies in Warmachine center around a warcaster, wizard that controls the armies steam-powered magic robots called warjacks, in addition to using spells that aid the army. Warcasters' roles will generally consist of buffing their controller's army, or throwing some damage at the enemy. Each turn a warcaster generates focus, which it uses for spell, or can allocate to warjacks. Focus can be used to generate additional attacks (note that unlike in Warhammer, units in this game only get on attack per weapon unless they have a role that says otherwise), or add an additional D6 to attack or damage rolls. If your warcaster dies, you lose regardless of the current situation, so most games revolve around killing your opponent's warcaster.
Warjacks, the machines controlled by warcasters, are usually the most powerful model in the army. They tend to be easy to hit, but their stats mean that it takes very high powered attacks to do anything to them. In combat, they generally also have the highest power weapons.
After warjacks and warcasters, the main models you use are units. They represent the main infantry of your army. They aren't as powerful individually as warjacks, but they are far more numerous. Units play any number of roles, from being aimed at killing other units, to killing warjacks, using magic to support your army, repairing warjacks, or killing your enemy's warjacks.
Factions
Warmachine has five factions, and Mercenaries who pretend to be a faction.
Khador
Cries of "For the Motherland", red banners, gold stars, frigid bitches, and beardy berserkers personify Khador. They're a northern Imperial Russia/Soviet Union style nation bent on imperialist conquest. Khadoran army tends to be smaller than other armies and usually slower, but their infantry and jacks tend to be much tougher and deal, more damage, which is kinda ironic since in real life Russia's tactics were the opposite. Their small armies and simple play style make them the best choice for beginners.
Cygnar
Cygnar is the most advanced nation, containing weapons that resemble WWI era tech and a ton of electricity firing toys, and the inventors of the warjacks. They're made to be a fairly nice place to live if not for the fact that Khador has it's eyes on their territory, the Protectorate of Menoth wanting to overthrow their government, and Cryx aiming to exterminate them and everything else on the mainland, or put simply, they'd be a nice place to live if the continent they were on wasn't a horrible place live. Their advanced technology is reflected in that they have the best ranged weapons in the game and place less aggressively than most other factions. Most of their units have the ability to shoot as well as fight, giving them something a combined arms feel. Like Khador, they're a good choice for beginners.
As a side note, Captain Maxwell Finn is a fucking beast.
Cryx
An island nation of pirates that was taken over by a Godzilla sized dragon named Toruk, the father of all dragons (dragons in Iron Kingdoms are basically gods, and Toruk is the oldest and most powerful). Toruk wants to rule the world, and aims to use the army he created in Cryx to aid in removing the other dragons, about the only thing that can kill him. Cryx is an interesting evil army, composed of pirates, undead pirates, ghost warriors, heavily armored liches, and demonic mutant witches. Their army is the fastest, cheapest, and largest on the table, and is for more advanced players. Cryx excels at assassinating enemy casters and utilizing "dirty" tactics, but is never quite as good in a stand-up fight. Many of their models also have abilities that kick in when they kill enemy models, and as per the tradition of an army with undead units, this includes adding models to their ranks.
Protectorate of Menoth
A bunch of religious fanatics that worship Menoth, the god of civilization. They were set in Cygnar, but the church of Menoth had some falling out with Cygnar since a more benign god named Morrow began gaining favor in Cygnar on account of Menoth being a bit of asshole. The Menoth church thus created its army, the Protectorate of Menoth, to forcibly convert Cygnar. Protectorate army is about setting up devastating combos, army synergy, and generally setting things on fire ("We put napalm on everything" can be their motto). Due to their issues with technology coming from Morrow, their army is far more low-tech and has ton of Gothic design to it. Many of their units include medieval knights and their warjacks' have similar designs to many machines in the Imperium of Man.
Retribution of Scyrah
A bunch of jerkass elves that nearly killed one of their main gods and think humans using magic makes things worse, so they want to kill all human mages. Their units are designed very differently and have more of sleek anime-esq look. They've got some interesting mechanics, like 'jacks with force fields that lose special abilities when they're crippled, dedicated mage hunting units, and ranged weapons with special effects. Most of their army is either jack-of-trades units or units heavily specialize in a certain role. However, they're certainly one of the harder and more expensive factions to play, limiting their player base.
Mercenaries
Once upon a time Mercenaries were meant only as additions to the real factions, so that two armies of the same faction wouldn't be composed of exactly the same troops. Since then, whinyfag mercenary fans forced PP to make a number of mercenary "contracts" to allow them to play weaker armies filled with dirty hobos. There are five mercenary contracts: Four Star Syndicate (which pretty much means playing any kind of mercenary), Highborn Covenant (they're French), Searforge Commission (they're Dwarves and thus the only viable Mercenary contract), and Talion Charger (a bunch of pirates. Lots of pirates. How many pirates can you imagine? There are more than that. Also, Napoleon. Also, a fukhuge cannon that is easily the best damn artillery piece in the entire game). As of MKII Mercs possess some of the most awesome warcasters, including Magnus the Traitor/Warlord who has the most awesome backstory for a character in the game and some of the most annoying abilities ever. Mercs are awesome because they get paid to get shit done and literally have access to a character for every rule breaking exploit, except for Protectorate Choir of Menoth, no one gets anything that broken.