The frost giants are the "giant" faction in the game. They do not train units with multiple members - all frost giants stand alone. A single frost giant is the equal of several men fighting together. They also gain perks on level-up, just like champions would. They also have a mechanic that, combat-wise, makes them similar to ordinary units - as they take damage, their attack will decrease.
Just as those who research the military tree unlock better formations such as 5-man squads and 9-man squads, the frost giants will unlock new cloaks that give them similar bonuses. So the player can choose the strength of the unit after what cloak the unit is wearing.
Frost giants use metal and crystal like other factions, but because they are trained solo (and thus wear fewer items) they produce 80% less crystals and metal from all sources (it's a balance thing).
Frost giants also gain +2 grain in any city connected to a Water shard.
Rivermod: Frost giants may not use magma forges in any way.
Overall, the frost giant faction is tilted towards army domination in the end-game. They are slow to train, but if left alone, will produce very strong units with their own perks from level-ups. They also have a large disadvantage to crystal and metal production in the early game (-80%), but once they have researched far enough up the military tree, the few of these resources they have gained becomes more valuable than for any other faction (since they still pay the same resources for equipment). Their +2 grain in any city connected to the water shards ensures very strong cities late in the game. They also have several traits that are specifically tailored to defeat an enemy focusing on the military tech tree - the most notable is that they have a weak Overpower trait that adds damage according to the size of the target unit. And finally, because they are solo in their units, magic and abilities that specifically kill army units easily (fireball etc) will have almost no impact on the frost giants. Frost giants take 50% extra fire damage, and 50% less frost damage.
The frost giants are weak against champions, and specifically fast and nimble champions with high dodge and dexterity. The frost giants' blows, that are devastating to army units, are easily avoided by the more experienced hero. Frost giants also have a very high cost for pioneers, making new cities and most importantly outposts outrageously expensive.