Most modern motherboard chipsets provide a USB host controller. Older machines which are not equipped with a USB host controller can be upgraded using a PCI cards with a host controller built in.
USB host controllers are compatible with either the Open Host Controller Interface (OHCI, by Compaq) or the Universal Host Controller Interface (UHCI, by Intel) standard. Both types have the same capabilities and USB devices work with both host controller types. Basically the hardware of UHCI is simpler and therefore it is cheaper, but needs a more complex device driver, which could cause slightly more CPU load.