Metamagic is the study of magical effects that manipulate or enhance other magical effects. These special enchantments are the building blocks of the metamagical code that drives Complex Enchantments and Magical Artifacts - two of the most highly-advanced branches of modern magical study.
The Artificer is the only class that normally receives training in Metamagic, but it may be available to other classes in some circumstances. It is not considered one of the Forms of Magic, but a separate category of enchantments that do not fall under any of the six established forms. When casting Metamagic, the modifier is your caster level.
These are enchantments that interact with other magical effects and change the way they work.
Components are special enchantments that represent pieces of larger complex enchantments. They don't have any effect by themselves but can be combined to create advanced magical machinery using the practice of artifacting.
The only non-enchantment spells that fall under Metamagic are interface spells. These are specialized spells created for the sole purpose of interacting with a certain enchanted object.