This post is a cop out. The first three sentences were added just to increase the ratio of content actually written by me. Nonetheless, I'm pretty sure someone else will find it useful.
Several times now I've wondered about the difference between the Linux ohci_hcd, uhci_hcd and ehci_hcd drivers. Well, at long last I looked it up.
The hardware that contains the host controller and the root hub has an interface toward the programmer which is called Host Controller Device (HCD) and is defined by the hardware implementer. In practice, these are hardware registers (ports) in the computer.
At version 1.0 and 1.1 there were two competing HCD implementations. Compaq's Open Host Controller Interface (OHCI) was adopted as the standard by the USB-IF. However, Intel subsequently created a specification they called the Universal Host Controller Interface (UHCI) and insisted other implementers pay to license and implement UHCI. VIA Technologies licensed the UHCI standard from Intel; all other chipset implementers use OHCI. The main difference between OHCI and UHCI is the fact that UHCI is more software-driven than OHCI is, making UHCI slightly more processor-intensive but cheaper to implement (excluding the license fees). The dueling implementations forced operating system vendors and hardware vendors to develop and test on both implementations which increased cost. During the design phase of USB 2.0 the USB-IF insisted on only one implementation. The USB 2.0 HCD implementation is called the Extended Host Controller Interface (EHCI). Only EHCI can support high-speed transfers. Each EHCI controller contains four virtual HCD implementations to support Full Speed and Low Speed devices. The virtual HCD on Intel and Via EHCI controllers are UHCI. All other vendors use virtual OHCI controllers.