OpenWrt is a Linux distribution and comes with own documentation, but much documentation is provided upstream, by the creators of the components.
RedBoot (Red Hat Embedded Debug and Bootstrap firmware) http://ecos.sourceware.org/redboot/
CFE (Common Firmware Environment) is a firmware developed by Broadcom. http://www.broadcom.com/support/communications_processors/downloads.php#cfe
This is the Homepage of the Linux Kernel: http://www.kernel.org/.
While all drivers belong into the kernel, official Wikis additionally exist for wireless and sound:
At the moment OpenWrt uses µClibc as C standard library. http://www.uclibc.org/
OpenWrt can be seen as a Linux Distribution for embeded devices. It does bring a Package Manager: http://code.google.com/p/opkg/
OpenWRT uses BusyBox to implement the shell environment and most of the usual Unix commands. Instead of having a collection of separate binaries, BusyBox condenses them into one. Executables like vi, ls and grep are merely symbolic links to the BusyBox binary. BusyBox Command Help
iptables, ip6tables, ebtables: http://www.netfilter.org/
OpenWrt prefers http://matt.ucc.asn.au/dropbear/dropbear.html over openssl-daemon because of its smaller footprint.
In order to handle various OpenWrt configurations, the dnsmasq init script is quite complex. Documentation on all the options passed to dnsmasq is available. Dnsmasq manual
Official documentation: http://www.samba.org/samba/docs/
Since 10.03 'Backfire' OpenWrt utilizes µHTTPd instead of httpd included in Busybox. https://dev.openwrt.org/browser/trunk/package/uhttpd/ (install
httpd (recompile busybox with this included)
Currently used by the X-Wrt project. lighttpd (install
lighttpd and mods)
mini-httpd and mods)