From the publisher's website:
Metamagic is a kind of give-and-take system for spells that allows spellcasters to gain benefits for spells in exchange for using higher-level spell slots to cast them. This gives a spellcaster considerably more flexibility, especially spontaneous spellcasters, who have a limited number of spells known but can add metamagic to spell slots on the fly. While most of the obvious spell enhancements (increasing some numeric value of spell game mechanics or removing some component to make casting easier) are covered in the Pathfinder Roleplaying Game Core Rulebook, those basic functions merely scratch the surface of what metamagic is capable of doing. The Genius Guide to Feats of Metamagic aims to expand the options into new areas, creating feats that changing how spells affect different targets, affect how a character uses all metamagic, and even weakening spells to lower their effective level.
New feats for spellcasters should mean more metamagic feats, to allow powers and classes (especially prestige classes) that depend on metamagic to gain new options and to increase the variety of builds for metamagic-using spellcasters. Just as monks, fighters, and rogues can use feats to adjust how they use their weapons, bards, clerics, druids, sorcerers, and wizards should have a wide range of options that let them modify how they use their spells. Spellcasters may wish to apply metamagic to focus their use of elemental spells (such as with Exchanged Spell), use metamagic to combine several commonly used combat spells into a single spellcasting action (with Cascade Spell), or explore a new kind of metamagic that allows them to cast a spell with a lesser spell slot by limiting some of its usual parameters (such as Weaken Spell). While there are a few feats that modify the use of all metamagic (such as a Metamagic Focus), the aim of this product is to help broaden the range of strategies spellcasters have for executing their spells so that is commensurate with the many ways nonspellcasters have to use combat maneuvers.