As stated at the OpenWrt home site:
OpenWrt is described as a Linux distribution for embedded devices.
Based on the Linux kernel, this distribution is primarily used on devices to route network traffic due to the fact that it is born because the Linksys released the source code of the firmware for their WRT54G series of wireless routers under the GNU/GPL license (that's why, the WRT into the name). Then, other chipsets, manufacturers, and device types have been included in turning the initial project into a valid and rock-solid software product.
OpenWrt's main components are the Linux kernel, the uClibc (or musl) C library, and BusyBox. All components have been optimized for size in order to fit into very small memory devices (bare but ...