Warcasters (sometimes shortened to 'casters) are a specialized type of battle mage found in Warmachine and the Iron Kingdoms setting, making up the majority of the protagonists, antagonists, movers and shakers of the metaplot. The vary greatly in function and form, however, as they almost always multiclass, running the gamut of classic PC archetypes; you have at minimum clerics, paladins, fighters, rangers and of course dedicated spell-slingers for each faction, lending to a whole range of playstyles just by swapping out 'casters.
Warcasters were originally introduced as a high-level prestige class for the Iron Kingdoms setting for Dungeons & Dragons 3.5 edition. Since then, they became the focal point of Warmachine, with an army literally built around the warcaster choosen to lead it. Additionally, most of the stories focus on the Warcasters themselves.
Although they are incredibly competent at fighting, commanding and using magic the main function of a typical 'caster is to command the warjacks of an army; multi-ton, steam-powered warmachines for which the game is named. Warjacks are hands down the toughest individual models you can find in the game, as well as the ones most able to dish out punishment at range and in their faces.
They do this via a Warcaster's unique mechanic; Focus Manipulation. With Focus, the performance of Warjacks (and the Warcasters themselves) reach incredible levels; Focus as a mix of Mana/MP and Fate points, typically 5-10 per warcaster, that is refreshed per turn. Warjacks can be 'allocated' Focus during the Control Phase of the controlling player's turn, and can then spend Focus to perform various shenanigans, from boosting their attacks and buying additional attacks to performing 'Power Attacks' (which are almost exclusively used by Warjacks, see their article). Always, a warjack that is topped up with Focus will utterly curbstomp an identical warjack that isn't given Focus. However, a typical warjack can only be allocated 3 Focus a turn (barring special rules that allow you to allocate more), so they aren't as broken as you'd think.
Linking your mind with a machine typically doesn't end in Mind Rape (at least, not in this setting), but it does influence the cortex/brain of the warjack, especially when a warcaster's been linked with a warjack for a long time, develops a particular affinity for it or just becomes that damn good at controlling warjacks. Because of this, several warcasters can form 'bonds' with their warmachines, adding on additional abilities that can make it even more scary to fight, first by letting more Focus be allocated to the 'jack, and by giving them additional effects while in their control range (which a Warjack shouldn't be out of); Major Victoria Haley of Cygnar, for example, gives her warjacks a movement boost (which can make a slow bro become a lightning bruiser from turn 1, or make an already fast warjack downright silly).