Most calling cards you will find now are global calling cards, for the most part.  I mean most cards have some limitations on them and will not let you call random countries.  For instance it may be hard to find a card allowing you to call some of the small countries in Africa or the small countries that now exist since the break up of the Soviet Union.  However, most cards are fairly global in the sense that you can call most major countries in the world.

The rates are obviously very different depending on which country you are calling and where you are calling from.  For example my parents live in the United States and it is much cheaper for them to call me in Venezuela than it would be if I was living in Vietnam or Cambodia.
Living in the global community that we do, most companies realize the need for people to call multiple countries throughout most of the world.  Many people have family traveling or living in the other countries.  It can be very hard to reach them by cell phone as cell phone service is sometimes limited, and expecting someone to use a landline to call long distance from a third world country is a little crazy.  For this reason global calling cards can come in very handing. These will allow you to call almost anyone anywhere in there world; forever keeping you in touch with your loved ones.

I personally have found global calling cards to come in handy on one more than one occasion and since you can now not only buy them in our local grocery or drug store, but also on the internet, they are also much more accessible and easy to get your hands on then they used to be.