They run on the Android (Google) phone OS, expect depending on the brand & cost any software version 1.6 - 2.2, how easy (or at all) it is to upgrade the software to later versions should be a factor in your decision 
I consider myself an optimist, albeit an optimist with cynical tendencies and a dark side that Lucifer himself would find a little creepy. (Perhaps you've noticed.) You don't normally associate cynicism with an upbeat pov. But I have exactly that combination and will defend it.
Never has so little had to be explained by me to so many so often.
also depends on what kind of user experience you want, for instance, if you want it to work just like a full blown pc, ie, running windows programs etc, then you`ll have to wait for the win7 tablets, at the moment they are very expensive, but the cost will start to drop in the coming year as they become more prolific, However, i don`t know how good the touch interface works on these devices.
android devices (& ios devices) have been designed from the ground up to be touchscreen controlled, & if you`ve used an android/iphone then you`ll pretty much be at home on an android tablet, as it`s the same os, as rass says, they`ll vary from android version 1.6 to 2.2 a lot of the cheapy chinese tablets ship with 1.6 (& have resistive screens... which i strongly recomend you avoid) but the more mainstream brands, ie, samsung/dell/advent have much better capacative screens (like the iphone, + they support multitouch etc, & are generally much much easier to use) & are more likely to ship with ver 2.1/2.2, & 2.3 in the future,
comes back to what you want out of the device, if you want to play embedded flash content, then you need a device running at least ver 2.2, but if thats not a major issue, then theres not a massive difference, & all will happily surf the web, play games & music/video
the cheapest (decent quality) 10" tablet at the moment is the advent vega, from dixons/pc world, £250, but it doesn`t have a cellular modem, so your restricted to wifi for web access, although a 3g enabled model is expected in the next couple of months. otherwise, theres the samsung galaxy Tab, which is a 7" tablet, with both wifi & 3g, but it`s around the £400 mark & thats with a contract, so thats an expensive option, in fact, you can get an ipad cheaper (£200 + 2yr £35 pm contract)..... because the new ipad model is out in a few months, so theres a stock dump ongoing.. iirc, orange & had the best deals.
hope this helps.
& i`ll add the dell streak, which is what i have, tho, i think your looking for a larger screen, the streaks "only" 5", but thats the sweet spot for me, between portability & funcionality.
Forever
A brand called Point of view are getting great reviews and the 7" although i would prefer a 10" uses android 2.2 and sells at a very reasonable £169 ish...
they look quite good, in fact the 10" tab looks like the advent vega (tbh, probably the same only under a different brand) but, just be aware, quite a few of th POV branded trablets are resistive touch panels, & they are no good, (think el cheapo £40 sat nav screen vs iphone screen)
www.overclockers.co.uk/showproduct.php?prodid=FS-000-PV
Forever
both are resistive touchscreens tho :( which is a major flaw :( & it`s a shame, becuase otherwise they would both be good tablets,
Forever
I bought a dead cheap tablet to play with from Amazon - the Eken M001 - because it`s very hackable. It`s not very practical for what I want because every time I want to use it, it goes through the full Android boot which can actually take longer than a Windows 7 boot. There seems to be no facility for hibernation/ sleep so the thing can do instant-on like my aging PDA.
Anybody with other models - can your tablet hibernate or do instant-on?
J Mark Oates
It`s Grand To Be Daft
sprockethole.myreviewer.com
Anybody with other models - can your tablet hibernate or do instant-on?
i thought they all worked the same way, basicaly the same as any android phone, with alterable screen time out etc, unless running video, it`ll blank out at about 30 seconds of inactivity, & go into standby, waiting for a call/text/email etc, it lasts without charge in this state for a few days (longer if celular modem/wifi is disabled)
& waking it up is as fast as a button press, & swiping your finger across the screen.
Forever