A Handheld Computer, or just "Hand Held", is a computer designed to be held in one hand and manipulated by the other hand.
A "personal digital assistant" (Personal Digital Assistant), or PDA, is a handheld designed to act as a personal organizer, containing an address book, schedule, to-do list, note pad, and other productivity tools. Such computers typically have a touch-sensitive LCD screen, an IR port, and a serial or USB "docking cradle" that can be used to exchange data with a desktop computer. Advanced models have wireless networking capabilities.
See also
Game Park (GP32)
and two wikis on PDAs: Handhelds Wiki and Sleepless Night Wiki
This (c2.com) Wiki already works (both reading and edit) on Pocket Pc's browser (PocketIE) with a Wi Fi or other wireless connection
Leonardo Da Vinci used to carry around a notebook everywhere to sketch, jot things down. It is an ancient and universal form factor; he would have probably used a Hand Held if he were alive today. The question is, will anyone's Hand Held become so famous in the future (be valuable because of the fame of its owner) as to be placed in a glass case in some museum for throngs to line up and see (as is Da Vinci's); it and its contents? Perhaps some as yet unknown scientist, author or artist. s/handheld/desktop/ and the answer is Tim Berners Lee
Andrew Wiles also carried around a paper notebook that helped him work out the solution to Fermats Last Theorem. What capabilities do Hand Helds need in order to be an effective replacement for such paper notebooks? One complaint with current Pocket Pcs is that in Notes and Pocket Word it automatically (can't be changed by user) draws dotted lines to group drawings making it confusing and harder than just writing on paper. In Notes you can't get rid of the ruled lines in drawing mode and, they are black which makes the drawing hard to see (again can't be changed and default ink is black. The ink can be changed but is a hassle and has to be re-set for each new drawing). Why not light blue like most ruled paper? There are 3rd party programs for drawing on these devices, but shouldn't freeform writing and drawing be the easiest to do right out of the box? New versions will probably fix this and Tablet Pcs might already be better (I haven't been able to test these in detail yet).
I came across a blog where the author got addicted to "experiencing" all the latest Hand Held with every little excuse, including Sharp Zaurus. This long 2004 post can be seen at www.geoffarnold.com 
. It seem to me his notebook is still needed.
Hand Held in your Mobile Phone
In my opinion, best arrangement is a mobile phone with 2.5G/3G (tri-mode GPRS if you travel internationally) and a Hand Held that can clip on to your belt (leaving pockets free for keys, phone, wallet etc), getting its wireless through Blue Tooth. Otherwise you have to pay for 2 subscriptions - one for phone, one for pda. You can also share the connection with your regular laptop if you need a bigger screen than a Hand Held. Like a Russian Doll set. Using only one device is difficult, as often you want to see the screen and can't while you are holding it to your ear. Plus the current PDA phones are too bulky. Earphones help, but there is a certain familiarity and convenience to just holding a phone to your ear. Earphones are best for driving, also; these should be Blue Tooth, because ones with wires get tangled. The ideal model would have both keyboard and stylus (I like Sony's new UX50), so while you are driving you can use it with one hand, but operate like "pen and paper" at other times. It should have SDIO card slots and GPS, and Wi Fi as well as Blue Tooth so you can use it at home or the office at higher speeds and without paying for airtime. Also at hotspots. Many new products have most of these features but not all in one package yet; for instance, the UX50 only has memory stick, no SDIO (if it did, you could add GPS, which it is also missing).
The new Bluetooth GPS devices let you add GPS to Hand Helds such as the UX-50 without a CF or SDIO Card. See www.socketcom.com
Handheld Computer that runs Windows Xp
Some have Tablet Pc like functions.
And all designed to trim your oversized wallet at the same time.
Dialogue Flybook at www.vnunet.com
OQO model 01 at www.vnunet.com
Sony Vaio model U50 and U70 at www.pdabuyersguide.com
Spotlight these days are on Ultra Mobile Devices (UMD) which have already sidelined the under performing Origami based Ultra Mobile PC (UMPC) market (e.g. the new Sony Vaio TZ). UMDs are closely related to Mobile Internet Devices (MID) and both have long battery life and designed for people with deep pockets.
See original on c2.com