The USB Host shield acts as an addon to the Arduino.
The USB Host Shield contains all of the digital logic and analog circuitry necessary to implement a full-speed USB peripheral/host controller with your Arduino. This means you could use your Arduino to interface with and control any USB slave device - thumbdrives, digital cameras, bluetooth dongles, and much more!
A four-wire serial interface is used to communicate with the host controller chip, so the shield connects the Arduino's hardware SPI pins (D10-13) to the MAX3421E. A USB type A female connector is wired up to the IC, and it also supplies 5V as any normal USB port would.
The Host Shield takes its power from the 'Vin' pin on your Arduino. Power from that pin is regulated to both 5V and 3.3V on the shield. All SPI signals are sent through a hex converter to step them down to 3.3V.