To understand what "EHCI" and friends are, you need to first know that every motherboard or PCI card that includes support for plugging in USB devices needs to have a USB host controller chipset on it. This particular chipset interfaces with the USB devices that you plug in to your system and takes care of all the low-level details necessary to allow your USB devices to communicate with the rest of the system.

The Linux USB drivers have three different USB host controller options because there are three different types of USB chips found on motherboards and PCI cards. The "EHCI" driver is designed to provide support for chips that implement the new high-speed USB 2.0 protocol. The "OHCI" driver is used to provide support for USB chips found on non-PC systems, as well as those on PC motherboards with SiS and ALi chipsets. The "UHCI" driver is used to provide support for the USB implementations you'll find on most other PC motherboards, including those from Intel and Via. You simply need to select the "?HCI" driver that corresponds to the type of USB support you'd like to enable. If in doubt, you can enable "ECHI," "UHCI" (pick either of the two, there's no significant difference between them), and "OHCI" just to be safe.