You can think of a network connection as being like a road. Some roads are wider, some are narrower, so each has a different capacity for how many cars can travel along them at any one time. The amount of information that a network connection can carry—how many cars—is referred to as the network bandwidth.
You will see bandwidth expressed in some variation of bits per second, like kbit/s or Mbit/s. Although this only tells you how much data is being moved every second; it doesn’t tell you if everything that was sent on the connection was actually received. To measure that, we have to talk in terms of throughput.
This is a measure of the amount of data that was sent successfully across the network and is arguably more important. A network might have a high headline bandwidth figure, but if the throughput is only half of that, then it falls short. Conversely, if a low-bandwidth connection has a consistently high throughput, then it could outshine it.
Most web hosting providers limit the network bandwidth for each user’s hosted websites, usually on a per month basis. Going over the allowance typically means that the offending site will be suspended, or the host will charge you (at a much higher rate than normal) for the extra network bandwidth.
Some hosts offer unlimited bandwidth hosting plans, which in some cases gives you an unlimited amount of data transfer per month. Usually the actual bandwidth usage, meaning the per-second transfer rate of a connection, is limited on these services, so that data transfer for the site stays at a reasonable level. With an unlimited bandwidth hosting plan, you’ll never have to worry about speed and usage caps.