The DD-WRT firmware is an open source alternative firmware for routers, wireless access points and modems. It replaces the firmware supplied by the manufacturer. (This process usually voids the warranty, though it is often reversible.)
It's focus is on improving security by adding admin privileges and on providing more wireless network options and SSH access. Power users can usually gain greater control over their hardware than by using stock firmware.
The emphasis lies on providing the easiest possible handling while at the same time supporting a great number of functionalities within the framework of the hardware used.
Use the tag if you have problems configuring your DD-WRT router or if it performs in an undesirable way. Please search the site for DD-WRT-related questions before asking your own.