Even though widely used in the field of networking, bandwidth and throughput are two commonly misunderstood concepts. When planning and building new networks, network administrators widely use these two concepts. Bandwidth is the maximum amount of data that can be transferred through a network for a specified period of time while throughput is the actual amount of data that can be transferred through a network during a specified time period.
Bandwidth can be defined as the amount of information that can flow through a network at a given period of time. Bandwidth actually gives the maximum amount of data that can be transmitted through a channel in theory. When you say that you have a 100 Mbps broadband line you are actually referring to the maximum amount of data that can travel through your line per second, which is the bandwidth. Even though the basic measurement for bandwidth is bits per second (bps), since it is a relatively small measurement, we widely use kilobits per second (kbps), megabits bits per second (Mbps), and gigabits per second (Gbps).
Most of us know from experience that the actual network speed is much slower than what is specified. Throughput is the actual amount of data that could be transferred through the network. That is the actual amount of data that gets transmitted back and forth from your computer, through the Internet to the web server in a single unit of time. When downloading a file you will see a window with a progress bar and a number. This number is actually the throughput and you must have noticed that it is not constant and almost always has a value lower than specified bandwidth for your connection. Several factors like the number of users accessing the network, network topology, physical media and hardware capabilities can effect this reduction in the bandwidth. As you can imagine, throughput is also measured using the same units used to measure the bandwidth.
As you have seen, bandwidth and throughput seems to give a similar measurement about a network, at the first glance. They are also measured using the same units of measurement. Despite all these similarities they are actually different. We can simply say that the bandwidth is the maximum throughput you can ever achieve while the actual speed that we experience while surfing is the throughput. To simplify further, you can think of the bandwidth as the width of a highway. As we increase the width of the highway more vehicles can move through a specified period of time. But when we consider the road conditions (craters or construction work in the highway) the number of vehicles that can actually pass through the specified period of time could be less than the above. This is actually analogous to the throughput. So it is clear that bandwidth and throughput gives two different measurements about a network.