Bandwidth measures the amount of information that can flow between two points in a certain period of time. The "broader" the bandwidth, the faster the information flow. You can use the term to describe how quickly information moves from the hard disk into memory, or from the computer to an add-in board on the expansion bus, or from one modem to another across a telephone line. Depending on whether the transmission is digital or analog, the rate is measured in bits per second (bps) or in hertz (cycles per second).
In an ANALOGUE communication, the difference between the highest and lowest frequencies that a transmission channel can carry: that is the band of frequencies it can pass, hence the name. In DIGITAL communication, the number of bits per second that can be transmitted through a communications channel.
The bandwidth of a particular part of your system can have a big impact on the speed of the system as a whole. Consider the situation when the computer needs to display a graphical image on the screen (all Macintosh and Windows screens are graphical). The computer's main processor is responsible for deciding which dots on the screen should be white, which red, and so on. But it must transfer that information through a bus to the video circuits that actually control your screen. No matter how fast the processor "composes" the screen; your monitor will seem sluggish if the bandwidth of the bus is narrow.
The newer expansion buses for the IBM and compatible pcs, the Micro Channel and the EISA, were developed partly to improve the bus bandwidth and so take better advantage of increased processor speed. And some new pcs have video circuits built right into the main circuit board of the computer, linked directly to the main processor via a high-speed local or direct bus.