There are several types of USB connectors out there, including some that have been added while the specification progressed. The original USB specification features Standard-A and Standard-B plugs and receptacles; the B connector was necessary so that cabling could be plug ended at both ends and still prevent users from connecting one computer receptacle to another.
The data connectors in the Standard-A plug are actually rested in the plug as compared to the outside power connectors. This permits the power to connect first, which prevents data errors by allowing the device to power up first and then transfer the data. Some devices operate in different modes depending on whether the data connection is made. For example, some battery-powered MP3 players switch into file transfer mode and cannot play MP3 files while a USB connector plug is fully inserted, but can be operated in MP3 playback mode using USB power by inserting the plug only part way so that the power slots make contact while the data slots do not. This enables those devices to be operated in MP3 playback mode while getting power from the cable.