This article will focus on Cisco switches running native IOS, not CatOS. For a page describing CatOS, please go here.
To reset a Cisco switch running native IOS, enter the 'erase start', 'erase-startup config', or 'write erase' commands. This WILL NOT change the boot variables, i.e. config-register or boot system settings. Here is an example of the output of the 'write erase' command.
Switch# write erase
Erasing the nvram filesystem will remove all files! Continue? [confirm]y[OK]
Erase of nvram: complete
System configuration has been modified. Save? [yes/no]: n
Once the switch finishes reloading, you will see a 'System Configuration Dialog' meaning the switch has returned to factory default.