Ironic is an OpenStack project which provisions bare metal (as opposed to virtual) machines by leveraging common technologies such as PXE boot and IPMI to cover a wide range of hardware, while supporting pluggable drivers to allow vendor-specific functionality to be added.
If one thinks of traditional hypervisor functionality (eg, creating a VM, enumerating virtual devices, managing the power state, loading an OS onto the VM, and so on), then Ironic may be thought of as a hypervisor API gluing together multiple drivers, each of which implement some portion of that functionality with respect to physical hardware.
The developer documentation provided here is continually kept up-to-date based on the latest code, and may not represent the state of the project at any specific prior release.