PDAs work mainly on stylus though some of the latest models come with a mini keyboard too. Blackberry phones comes under PDA. The PDAs recognize handwriting, voice recognition. Many models have loads of softwares bundled with the PDA.
Phones like Nokia E71, Palm Treo Pro, HTC Touch Pro, imate Ultimate 8502 are some of the top PDAs that work more like a mini laptop. These phones help in connecting to the Internet easily as they come with wi-fi that helps the owner to check mails, and web wherever they are. As these phones comes with external storage memory, it is easy to use the phone as an externel storage device also. Most of the phones help the user to view and edit documents or spread sheets. These PDAs also comes with regular mobile features of music player, media player, camera, games too.
Depending upon the features that comes with the PDA the price also varies and unless and until it is absolutely necessary, it is not wise to go for PDAs. But it is the best friend for any one who are constantly on move and who have most of their detail stored virtually and who use less paper and more electronic media.