Help!!~ want to enquire something about the mobile network~~

Hi everyone, firstly, my English is not good~
My phone is samsung galaxy 2. When I active my data network mode and log on to the internet, the signal of the net can be '3G', 'H', 'E', 'G'... do you guys know what does it mean??
I want to ask because my sim card is voda, and they charge me a lot more than my $49 Cap... Someone told me that they'll charge me if my phone is under roaming. So I want to know that when it's under roaming circumstance, what can be shown on my phone???
Thanks for all help~~

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'3G', 'H', 'E', 'G' i think this is the detail information of your mobile