PalmOS is the OperatingSystem for the Palm? handheld computers.
PalmOS has been re-written a number of times. The current release for most devices is 5.x, although version 6.x has been released but nobody really seems to want to use it in their devices.
Palm? (or more accurately one of the two Palm companies - which I won't document as it looks like it is about to change yet again....) has now bought a Chinese Linux company and says it will migrate PalmOS to Linux. This will be done by PalmOS sitting on top of Linux and old applications will go through this layer.