I have an android tablet that I would like use on holiday.
I have no idea if a bog standard mobile dongle will work or if I need to get a specific one.
Also, what does ‘rooting’ or ‘rooted’ mean? Do I need this?
Most tablets will take a usb memory stick. All require an adapter. Why do you need one? What is the make of your tablet. Rooted or rooting is way of having total control of your device…not always a good thing unless you know exactly what you are doing. You can access system files etc.
Can you post the tablet Make/Model/Android version.
If you are holidaying in the UK (and assuming your device has WiFi) then you could try 3 MiFi. This I find is a better option (compared to the USB dongles) as you do not need to connect it direct to your device but acts as a Wireless Hotspot. No need to worry about firmware/drivers.
You can also connect up to 5 devices simultaneously.
If you got a family with several WiFi enabled devices then this is godsend (I can vouch for that) 
Oh yeah … why didn’t I think of the MiFi :-[
You can get unlocked ones on ebay … such as:
http://www.ebay.co.uk/itm/UNLOCKED-HUAWEI-E5332-3G-MiFi-Mobile-WiFi-Broadband-Router-with-3G-TS9-Antenna-/310674848323?pt=UK_Computing_Mobile_Broadband_Devices&hash=item4855a9fa43
The MiFi connects to the mobile network, and then acts as a wireless router … so you can connect multiple PC’s to it (as long as they have wireless).
Think of it as a battery (or USB) powered wireless router that connects to the mobile network, rather than the wired telephone network … because that’s exactly what it is.
Argos also do a Vodafone MiFi:
http://www.argos.co.uk/static/Product/partNumber/1168044.htm?CMPID=GS001&_$ja=cgid:7388166782|tsid:41408|cid:152481902|lid:52148557734|nw:g|crid:27922763342|rnd:1994899221757685812|dvc:c|adp:1o2
Yes, sorry, I was thrown a bit when you said dongle and rambled on about usb. Yes the MiFi is good I use the Three network with my mobile as router and it works great but…You say you are going on holiday. If it is abroad be careful of data roaming charges. The hotel WiFi would be better. At home the MiFi would be good. What are your reasons for wanting it? There may be other options.
now t-mobile is part of EE there mifi works best where i live (east devon)