PDA is a short for Personal Digital Assistant. They are handheld computers that were originally designed as personal organizers, but has become much more versatile over the years. These devices contain much of the functionality used with a personal compoter (PC), but are much smaller than a typical desktop.
But most importantly, at least from a mobile device manufacturer's viewpoint, lots of PDAs are also mobile phones. It's worth noting that the main difference between a PDA and a smartphone is that a PDA is data-centric with some phone functionality as opposed to a smartphone, which is voice-centric with some PDA functionality.
Typical PDA features are touch-screen, memory cards, connectivity, synchronisation with home computer or corporate servers.
Major PDA operating systems are: