This documentation will give an overview of how the v0.1 libusb API works and relates to USB. Work is rapidly progressing on a newer version of libusb, to be v1.0, which will be a redesigned API and is intended to obsolete v0.1. You may want to check the libusb website to see if it is stable and recommended.
This documentation assumes that you have a good understanding of USB and how it works. If you don't have a good understanding of USB, it is recommended you obtain the USB v2.0 specs and read them.
libusb is geared towards USB 1.1, however from the perspective of libusb, USB 2.0 won't be a significant change for libusb