Components are the building blocks of complex enchantments and magical artifacts. Although the term usually refers to Metamagic components, any spell or enchantment can serve as a component in certain circumstances.
Metamagic components are denoted by the Component keyword. These enchantments don't have any effect by themselves, but serve as building blocks to combine other spells and enchantments together for more intricate effects.
A complex enchantment generally has at least three components: an input to receive information from the environment, a controller to make decisions or modifications to that information, and an output that determines the final effect. More complex enchantments can have many more components that create multiple branching logical paths and states for an artifact to take.
Input components can be anything that provides information. Metamagic components with the Input keyword serve as the starting-off point to most artifacts' logic, but any spell that provides the caster with information can also be used as an input, for example Detect Life.
Controllers are always Metamagic components. These are enchantments that sit between the input and output, functioning as logical steps that modify how the enchantment works. It's possible for an enchantment to have multiple controller components that create many branching paths and possible effects.
Output components are simple enchantments or spells that generate the effects of the enchantment. These can be any spell or enchantment.
These special components change the way other components work. Not all complex enchantments or artifacts use them. When present, their effects override one or more effects of another component, such as making an Input more sensitive or causing an Output spell's effects to only work on certain creatures.