A Universal Serial Bus (USB) is a bus standard for connecting peripherals to your PC.
Up until fairly recently, motherboards were based on the original USB 1.1 standard. This offers users data transfer rates of 12Mbps and is used to connect low to middle bandwidth devices such as scanners, printers, keyboards and mice to a PC. A single USB port can be used to connect up to 127 peripherals. The USB standard also supports plug-and-play installations and hot plugging (plug-and-play is the ability to add and remove devices to a computer while the computer is running and have the operating system automatically recognise the change).
USB 1.1 has been now superseded by the backwards-compatible USB 2.0 (also referred to as "hi-speed USB"), which can offer data speeds of up to 480Mbps - 40 times that of its predecessor. USB 2.0 supports higher-bandwidth devices, such as cameras, next-generation scanners, video conference cameras and fast storage units. USB 2.0 should look the same as 1.1 from a user's point of view, with the only noticeable difference being faster data transfer from peripherals.
A motherboard will generally have four to eight USB ports, one serial port and one parallel port. Again, motherboard manufacturers can build different numbers of USB ports onto the board.
IEEE-1394, otherwise known as FireWire or i.Link, is a serial bus designed to connect devices to your computer -- similar, in practice, to USB 2.0. This peer-to-peer interface allows you to connect up to 63 devices to one another, without the need for a PC and is particularly suitable for high-performance applications and for connecting high-end consumer devices such as an external DVD burner, or a digital video camera.
Most new motherboard now come with between 1 to 3 FireWire ports, which can be used by connecting a special D-bracket, which is usually supplied with the motherboard, to an internal pin header on the motherboard. Some budget motherboards do not have FireWire at all though, so make sure you check beforehand if this is going to be a feature you require.
The original FireWire standard offered data transfer speeds of up to 400Mbps. A new version of FireWire, called FireWire 800, offers data transfer speeds of 800Mbps, making it twice as fast as the existing FireWire 400. The new standard, which was released in early 2003, is backwards compatible with the older version so you can still connect FireWire 800 devices to existing 400 ports, although they will only run at the slower speed.
Several industry players are now referring to the older 400Mbps FireWire as "FireWire A" or "1394a", and the new 800Mbps standard as "FireWire B" or "1394b", so you may see FireWire-based devices described using these terms. At this stage however, only some flagship model motherboards have the newer FireWire standard.