Well there is actually a very BIG difference, usually misunderstood as well.
Throughput is the amount of information (in bits, bytes, packets, etc) that can go or be sent through a connection or conductor in real life situation. Things that can hit it are bad connectors, bad cables, noise, or even collisions and the size of the packets being sent etc. All these things can hit and impact the throughput of the circuit..
Bandwidth on the other hand, is the amount of data that can be physically transferred through the media of choice. Like 100mbs for Cat 5 cable. Its the scientific calculated unit of what should be possible to send. In real life, that number may well not be reached because the actual throughput is not available.
They are proportional but really one is theoretical (bandwidth) and the other is real (throughput).
ISPs always sell the "Bandwidth" but they forget about the throughput because they can't know or even really estimate that number as there are too many variables. That is why most of them say "up to 3Mbs" notice the up to.. the real throughput will be lower and that is real.