The Arduino USB Host Shield ows you to connect a USB device to your Arduino board. The Arduino USB Host Shield is based on the MAX3421E (datasheet), which is a USB peripheral/host controller containing the digital logic and analog circuitry necessary to implement a full-speed USB peripheral.

The following device classes are supported by the shield:

HID devices: keyboards, mice, joysticks, etc.
Game controllers: Sony PS3, Nintendo Wii, Xbox360.
USB to serial converters: FTDI, PL-2303, ACM, as well as certain cell phones and GPS receivers.
ADK-capable Android phones and s.
Digital cameras: Canon EOS, Powershot, Nikon DSLRs and P&S, as well as generic PTP.
Mass storage devices: USB sticks, memory card readers, external hard drives, etc.
Bluetooth® dongles.

Arduino communicates with the MAX3421E using the SPI bus (through the ICSP header). This is on digital pins 10, 11, 12, and 13 on the Uno and pins 10, 50, 51, and 52 on the Mega. On both boards, pin 10 is used to the MAX3421E. Pins 7, 8 and 9 are used for GPX, INT and RES pins.