A factory reset is the ultimate cleansing of your Android device. It's usually either a last resort to fix a problem, done before you sell it, or because you like to flash ROMs. When you perform a factory reset you're essentially wiping out everything you've ever done to the phone and restoring it back to the basic manufacturer software. As we've mentioned before, it doesn't uninstall any software updates you've received from the folks who made your phone, but it does wipe out any core application updates you've grabbed from the Google Play store. The technical details are as follows:
- /system is untouched, because it's normally read-only
- /data is erased
- /cache is erased
- /sdcard is untouched
When your phone or tablet reboots, it's like it was when you opened the box as far as apps and user data goes, except for your data on the SD card partition (either a real, physical microSD card or a partition named sdcard).
Doing a factory reset is easy -- open the settings, do a little digging (different manufacturers put it in different places, but start with privacy or storage), select it and confirm. Your device will reboot into the recovery partition, erase everything, they reboot into the setup again. One thing to note though -- if you've rooted and ROM'd in any way, you should never do a factory reset from settings. Often times it works just fine, but some devices and some ROMs are so different once hacked that you'll end up with a bricked phone. We hate bricked phones around these parts. Follow the instructions from the folks who developed the software you're running instead, and use the reset method they recommend.