APN - Access Point Name

APN - Access Point Name. This is the name of a service running on a machine that gives your 3G modem access to the Internet. This machine is located somewhere in your Service Providers network. Sometimes you also need to know a user name and password. Many Service Providers have several APNs. You have to use the correct APN for your SIM. If you do not know which APN to use then ask your Service Provider. 

There are several APN lists on the Internet but this is a good one:

http://www.quickim.com/support/gprs-settings.html