I ordered a Garmin Street pilot C340 which said that i came loaded with maps of US.....?

Usually you get a base map with the product that contains all major roads and highways etc. The SC card is usually there for you to add on more detailed or specific information that you can purchase or subscribe to that is more specific to your use of the GPS. One item is much more detailed street information for a State or group of States that shows every street and also may show tourist attractions, motels, and other items of interest.