A factory reset results in a full reset of an Android device to the original settings of the currently-installed ROM. This process deletes all user data and user installed apps, so the device looks like it came fresh out of the box. Usually this corresponds to the state it comes in from the factory (hence the name "factory reset"), which sometimes can be a little misleading: A factory reset will not roll back any possibly applied updates to the system.
- hard-reset: this does not intend to delete anything (though some data might be lost, if they were not saved). If you cannot safely shutdown your device, but somehow have to power-off, a "hard reset" takes care of this by cutting off the power supply -- usually by removing the battery or (for devices where it is not removable) pushing the reset switch.
- soft reset: like Ctrl-Alt-Delete on your computer, some devices offer a similar key-combination to trigger a shutdown in case the touch-screen is no longer responding