The 3G cellular network provide your mobile with high speed internet connection, which is ideal for exchange of continuous data such as the updating of applications, geolocation, real-time apps..etc
The majority of these apps work in the background, consuming the phone battery and increasing data charges. In the case that you want to limit your data charges or save some battery, Android allows you to disable the 3G option.
The procedure is as follows:
Go to "Settings > Wireless & networks > Mobile networks
Uncheck "Use Packet Data".
Note that: Once 3G is off, you can still connect to the internet via EDGE network, which is slower.
There are several widgets allowing your to Enable/Disable 3G directly from your Homescreen