Reset And Reboot
From DD-WRT Wiki
This page is an attempt to clarify the different resets and reboots. You'll find other related and valuable information at Factory Defaults.
 Hard Reset ("30 reset")
A hard reset sets the router back to the firmware's defaults, thus it will clear all the settings you have configured in the router. Here are some of the different ways that users are doing hard resets. Those methods will not unflash your router to the default firmware. They will reset to defaults such settings as IP address or password.
After a hard reset the router will take a few minutes to boot
- Press and hold the button while the router is on, and keep holding it about 30 seconds. On different models you may see rapid flashing of an LED, or a red error or diagnostic LED. Wait for it to return to normal operation (typically power-LED on solid). Normal behaviour here is for it to not actually clear the nvram, although many people think of it and phrase it that way. What it is supposed to do is return all settings to factory state, or to say it another way, to firmware defaults. If you added new non-factory variables they should still be there after this type of reset. One way of tracking the status of what's happening while holding in the reset button is to leave a network cable plugged in to your computer and the router, then watch the screen on your computer. After the network has lost a connection for the second time, the router is usually reset back to firmware defaults.
If above method did not work at all it probably means resetbuttond is set to disabled.
 Hard Reset ("30/30/30 reset")
Holding down the reset-button while plugging in the router achieves a different goal than the thirty second reset. Here the bootloader is in charge, so getting it to clear the memory for you may have different results. Some platforms will completely empty the nvram and depend on another stage of the bootloader or firmware to repopulate it. On some less-supported hardware this may have unpleasant results, so use the following two reset methods cautiously.
To do a "hard reset for anyone not knowing OP or otherwise, hold the reset button with the power on, for 30 seconds, then while continuing to hold the reset button, unplug the linksys router, continuing to hold the red button in the back for 30 seconds, and while still holding it in, plug in the power again and keep holding the reset button for an additional 30 seconds. This will wipe the nvram out. Keep in mind with hard resets the linksys routers can be touchy I had to do it three times till it fully cleared out the nvram on mine" I think this forum poster actually meant that it will reset back to firmware/factory defaults, but as stated above, it could have unpleasant results "on some less-supported hardware".
 Cold Boot
A cold boot is when one remove power from a device, for a long enough period of time to ensure that all ram/chip registers are fully cleared. Ram has been known to be readable for minutes after a power off, and methods can be used to read said ram for hacking purposes. Chips have been known to maintain state for minutes as well.
Regardless, ram is supposed to be entirely cleared on boot, yet there are various reasons why you can not be positive that this is the case. Design flaws, low cost design methodology, bugs in ram chips, can all result in ram or logic gates in chips, not being cleared on boot.
A cold boot ensures that the ram and other various chips are all at zero state upon boot. Again, while the state of logic gates should be cleared on boot, this may not be the case.
For a cold boot, leave hardware off .. unplugged from any power source, for 30 seconds.
It should be noted that this will not clear the nvram or set the router back to factory/firmware defaults.
 Clearing The nvram
"To clear or wipe out the nvram" is a term commonly used to indicate "reset the router back to the firmware's defaults", although the two terms do not mean exactly the same thing. Many users refer to "factory defaults", but in reality it's "firmware defaults" because you'll revert to DD-WRT's defaults if you have DD-WRT installed.
 Power Cycling
Some users refer to "power cycling" a router. Power cycling is simply rebooting the router, but usually by unplugging the router and then plugging the it back in. Some users claim to get different results by leaving the router "unplugged for a while".
 Login (password) after Reset
After a reset of DD-WRT, the login gets reset to: user "root", password "admin".