DD-WRT is a third party developed firmware released under the terms of the GPL for many 802.11g wireless routers based on a Broadcom chip reference design. For a list of supported devices, please see the Supported Devices page.
Which means that it is an alternative firmware for those Wireless Routers listed in it's supported device list. By replacing the firmware on said routers you can open up the offerings that the router has initially. There are so many tweaks and alternate uses that DD-WRT gives your router it is amazing. Personally, since I have been using DD-WRT on my routers I have managed to not only improve the performance of the router, but make signal improvements across the whole wireless network I have made. Most firmware will not give you the options to tweak out the router that DD-WRT does and NO firmware that I have seen allows you to change the purpose of the router (i.e. bridge, repeater, etc...). The latter benefits of DD-WRT really lets one expand their network without sacrificing performance throughout.
I can't recommend DD-WRT enough...by all means check it out!
- DD-WRT WIKI
- DD-WRT Hompage