As a spellcaster's knowledge of magic grows, she can learn to cast spells in ways slightly different from the ways in which the spells were originally designed or learned. Preparing and casting a spell in such a way is harder than normal but, thanks to metamagic feats, at least it is possible. Spells modified by a metamagic feat use a spell slot higher than normal. This does not change the level of the spell, so the difficulty class for saving throws against it does not go up.
Wizards and Divine Spellcasters: Wizards and divine spellcasters must prepare their spells in advance. During preparation, the character chooses which spells to prepare with metamagic feats (and thus which ones take up higher-level spell slots than normal).
Sorcerers and Bards: Sorcerers and bards choose spells as they cast them. They can choose when they cast their spells whether to apply their metamagic feats to improve them. As with other spellcasters, the improved spell uses up a higher-level spell slot. But because the sorcerer or bard has not prepared the spell in a metamagic form in advance, he must apply the metamagic feat on the spot.
Effects of Metamagic Feats on a Spell: In all ways, a metamagic spell operates at its original spell level, even though it is prepared and cast as a higher-level spell. Saving throw modifications are not changed unless stated otherwise in the feat description.
The modifications made by these feats only apply to spells cast directly by the feat user. A spellcaster can't use a metamagic feat to alter a spell being cast from a wand, scroll, or other device.
Metamagic feats cannot be used with all spells. See the specific feat descriptions for the spells that a particular feat can't modify.
Multiple Metamagic Feats on a Spell: A spellcaster cannot normally apply more than 1 metamagic at a time to a spell, however at epic levels a caster may select the Auto-silent/still/quicken feats. While these feats apply meta-magic effects they do not infringe on the 1 metamagic per spell limit (e.g. it is possible to create a still, quickened, silent & empowered spell if you have selected the appropriate level of auto-still, auto-quicken and auto-silent, before then adding the empower metamagic). You can't apply the same metamagic feat more than once to a single spell.
Counterspelling Metamagic Spells: Whether or not a spell has been enhanced by a metamagic feat does not affect its vulnerability to counterspelling or its ability to counterspell another spell.