http://www.beyondlogic.org
especially
http://www.beyondlogic.org/usb/ftdi.htm
which says almost all you need, i.e. the best way is to convert USB to UART which is simple to connect to '51.

JW


PS. USB is not peer-to-peer, i.e. there is always a "master" (called here HOST) and "slaves" (called DEVICES). Typically, HOST is a PC; DEVICES are then for example an USB camera, USB "memory stick", USB keyboard. Although not impossible, it's hard to implement a HOST with '51, so, don't expect to connect a DEVICE (such as USB camera, USB "memory stick" etc.) to '51 via USB.