Tutorials are very good for learning about how usb works. The usb article in wikipedia is a good place to start. The beyond logic website has tutorials and information on ftdi chip, philips usb microontrollers, and general usb tutorials. Jan axelson usb central site is a excellent site for information, material, hardware, and software for usb.

Universal Serial Bus (USB) is a serial bus standard to interface devices. A major component in the legacy-free PC, USB was designed to allow peripherals to be connected using a single standardized interface socket and to improve plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping). Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.

The home of the USB in a nutshell tutorial - this six part acticles talks about electrical, standards, standards. A very good source for learning USB. There are lots of resources for USB 1.1 IC's from manufacters such as National Semiconducter, Motorolla, Microchip, Cypress, ftdi.............. and more.

Jan Axelson's USB Central is a great resource for USB tutorials and articles. This website covers electrical, connecters, protocol, USB development boards, USB Data aquasition, USB to i2c, USB on the Go, wireless USB, specifications, standards, drivers, hosts, articles and books.

A Wiki page dedicated for PIC18F USB device development. There is some tutorials regarding PIC18F based Usb Bootload, HID Mouse moving, HID Mouse buttons & wheel, HID KeyBoard, CDC and COMx port emulation. If you are using PIC, then you can save your time.