If you want information on an area code, the fastest way I've found is to go to Wikipedia and search for the string 'Area code' + the area code for which you want information. E.g. Area code 902.
Most of the area code pages also give information on which telephone exchanges correspond to geographic areas.
In many cases, the page will also give a lot of useless, though interesting, trivia on the history of the area code -- e.g. 902 was once shared by four Canadian provinces, but now is only shared by two.
If you've got some time to burn, you can try to understand the E.164 standard for telephone numbers (*shudder*).