Rigify is a bundled addon that provides ready made rigs (armatures) with custom controls. It is designed to help build full character or creature rigs with a building block approach. Use this tag for questions about creating and using meta-rigs, and questions about generating, parenting, and using control rigs.
The Blender manual describes rigify thusly:
Rigify helps automate the creation of character rigs. It is based around a building-block approach, where you build complete rigs out of smaller rig parts (e.g. arms, legs, spines, fingers…). The rig parts are currently few in number, but as more rig parts are added to Rigify it should become more and more capable of rigging a large variety of characters and creatures.
Rigify uses a two part system. Metarigs are deformation armatures that are meant to be positioned to align the rig to the mesh of the character or creature. Control Rigs are generated from metarigs by the generator.
Once generated, control rigs are parented to the character mesh and can be used in Pose mode for animation.
Use this tag for questions about creating and positioning metarigs; generating and parenting control rigs; and operating generated control rigs.