Help me settle an argument before this customer rings me back

If you have 1 Megabit ADSL you have 1024 (dont get me started on mibi and mega) bits per second 1 Megabyte per second would be an 8 Megabit line.

Throughput is limited by latency to some degree. There is also an overhead caused by the underlying protocols.

Or you could blame it on your crazy accent. "Bite, yes - b-i-t spells Bite. B-Y-T-E? That spells Beet! duh."

The actual speed vs. theoretical speed is limited by all sorts - upstream speed of the server you're trying to download from (if all it can do is 1Mb then that's all you're going to get regardless of your speed). It's also limited by what else is going on on the line at the same time, so if the computer you're downloading from has 1Mb upstream and two people are trying to download from it then you're only going to get 1Mb between the two of you, assuming both of you can download >=1Mb's worth.

This all started (in my reckoning) back in the '70s/'80s when a modem would run at 400 baud, i.e. 400 bits/second. When that got to 1000 baud, it was 1Kb/s and so on; it's extended to megabits. It's confusing, since memory is dealt with in bytes and always has been. It makes me wonder now that I think about it whether or not 8Mb actually means 8 mega-baud, i.e. 8 mega bits/second.

Your network connection being 100Mb is basically the local area network connection - this is prior to it being throttled by your connection to the internet. That's to say, copying to another machine on your own network could run at 100Mb (assuming no other traffic etc.) but won't if you're getting stuff off the internet.