There are a number of different types of resets that can be performed on a Palm OS device:

A soft reset is the equivalent of 'rebooting' a computer. The OS restarts, and the user data is not lost, perhaps with the exception of any unsaved data. This is normally achieved by inserting a pin or the tip of the stylus into the reset hole on the back of the device. A soft reset is most useful when a program has caused the device to freeze and become completely unresponsive to other inputs.

Tip: on some devices the reset hole is too small for the tip of the stylus, but in most of those cases the end of the stylus can be unscrewed to reveal a pin inside that is small enough to perform a reset.

A hard reset is equivalent to a full factory reset, and causes all user data to be wiped from the device. To perform a hard reset, hold the power button down, and press the reset button on the back of the device once momentarily. The screen should display a message about erasing all data on the device. Release the power button. To continue with the hard reset, now press the up-scroll or up-navigation button on the device. The RAM and Flash memory on your device will be completely erased. (*Exception: To fully erase the hard drive on a LifeDrive device, an option is presented to perform a longer Secure Erase by pressing the down navigation button instead.)

A warm reset is equivalent to restarting into 'safe mode', with system extensions (such as HackMaster hacks, OS patches, Alarm settings, etc.) disabled. All records and entries stored in the handheld device are retained with a warm reset.

To perform a warm reset, hold down the up-scroll or up-navigation button on your device, and press the reset button on the back of the device once momentarily. After the device boots, you can release the up button.

This can be helpful if some software is causing the Palm OS system to crash or 'boot-loop' when starting up. Performing a warm reset will often bypass the 'boot-loop' error, allowing you a chance to remove any third-party application that have recently been installed that could be causing the problem. Note: some hardware features may also not work in this mode (e.g. bluetooth).

After the warm reset has been used to resolve an issue, a regular soft reset should be performed later on to reactivate all the system extensions and hardware features again.

An extended hard reset is like a hard reset in that it will completely erase your handheld, however it will skip the 'first-run' initialisation of onboard software programs. As a result, all of the 'default' sample database values are not recreated. For example, the sample doodle in the Notepad application is not recreated in this mode.

To perform an extended hard reset, start by performing a hard reset, by holding down the power button and pressing the reset button momentarily. Once the message on the screen is displayed about erasing all data, now press and hold the up-scroll or up-navigation button. While still holding down this button, press each of the Datebook, Address Book, ToDo, and Note/Memo buttons once. Now release the up button. The device will then perform an extended hard reset.

Pressing some but not all the buttons while holding the scroll key will result in a normal soft reset. Pressing no buttons while holding the scroll key will result in a normal hard reset.

This differs from a normal hard reset, which will automatically create the databases for the built-in applications, and will insert some specimen records into those databases: numbers for Accessories and Technical Support will be created in your Address database, a registration reminder will be created in your ToDo database, and several helpful memos will be created in your Memo database.

In addition, a normal hard reset will also create a Network database with some default Network profiles for your Network panel in Prefs, and a file of several default MIDI alarm sounds will be created.

If you have the Notepad application, a normal hard reset ensures a sample note will appear on your NotePad, and if you have the Mail application, an email message from 'Mail Help' will be automatically inserted in your handheld Inbox. An Extended Hard Reset will ensure that all of these startup files are not created.

If you decide to perform an Extended Hard Reset and install one or more third-party applications that expect to see the built-in databases present, you may encounter an error. For this reason, it's suggested that after an Extended Hard Reset, you run each of the built-in applications (Datebook, Address, ToDo, Memo, and if installed, NotePad and Mail) at least once to ensure empty built-in databases are created. After an Extended Hard Reset, you will not be able to select alternate alarm sounds for Datebook alarms, but the default alarm sound will still be heard for appointments with alarms set.

Credit to VOTIT for information regarding the Extended Hard Reset, and thanks to Tavisco for linking me to this.