A 3G key, pronounced "Clay trwa j" will allow you to connect to the internet wherever you are. In theory !! -- it looks like a USB memory key and fits the USB port on your computer. This should allow you to use mobile broadband wherever you are. A couple of things to watch out for though;
Is there good 3g coverage where you wish to use the computer? If not, it will be a frustrating experience trying to connect to the internet, rather like trying to find a mobile phone signal.
3G keys are unsuitable for people that have large download or upload needs. You should check the terms of the contract regarding data transfer limits. The key will automatically cut you off if you overrun your monthly limit.
Using these keys can be costly. Check the price per hour.

Wherever possible a traditional ADSL line in your house is by far the best way to connect to the internet.