Conjuring is a mod about working with souls and other strange substances found in the Nether to create contraptions that are all somehow concerned with the conjuration of entites. The two main innovations that spring from this are the Conjurer and the Soul Alloy Tool Set. Have a look at Images to see them in action.
Conjuring is fully configurable, has thorough documentation in form of the Conjuring Enchiridion and aims to be well balanced even for medium-sized modpacks with it's default setup. Playing with REI is highly recommended as not all custom crafting methods and their recipes are outlined in the Enchiridion.
As Conjuring 2 is currently still somewhat in beta, make sure to get a backup of your worlds before you install the update.
The Conjuring Enchiridion is the in-game guide-book that covers every aspect of the mod. It has been crafted with great care taken to be both an entertaining read as well as just highly informative. To obtain it, throw together some Soul Sand and a Book in a crafting table.
Beware that Conjuring also features a swathe of easter-eggs, none of which are outlined in the Enchiridion.
The Conjurer is a basic, yet surprisingly powerful modification of the Monster Spawner everyone is familiar with. It can be configured to summon most entities you could dream of and can be upgraded in all relevant aspects like speed, amount of mobs spawned, required player distance and so forth.
The Soul Alloy Tools are a set of end-game tools with secondary abilities focused on being balanced yet fun to use. They can also be upgraded with the same Aspect Gems the Conjurer uses to either add more abilities or to upgrade basic stats like mining speed or the level of Looting.