What is the difference between co-lo providers that charge on the
basis of bandwidth in what I understand to be the meaning of the word
(i.e. pipe bandwidth - in terms of speeds such as 10Mbps and 100Mbps),
and those that charge on the basis of "bandwidth" in the sense of the
amount of what I can only describe as throughput in terms of Gb per
month. Why the difference in terminology, and how can providers
provide an "unlimited" amount of bandwidth from in the former meaning
if the providers themselves are being charged per Gb throughput from
their wholesalers?

"how can providers provide an "unlimited" amount of bandwidth from in
the former meaning if the providers themselves are being charged per
Gb throughput from their wholesalers?"

Simple!

Here is an example:

You propably might have seen dial up plans with Unlimited bandwidth.
Well lets do a calculation:

A 56Kbps Dial Up internet connection transfer at averaging 5KB per
second, in theory you would be able to transfer 12.65GB of data
monthly, but in reality, including all the drop outs, varying speeds
etc.... you would mostly be downloading at about 1GB a month at an
average.

Where as, if you were given a 100Mbps you would be able to download
~12MB! That is damn fast and you would be consuming a lot of
bandwidth, so they charge you in terms of limited bandwith in this
case.

Its basically a marketing evilery.

Cheers

I'm guessing that was meant to be "~12GB" rather than "~12MB"? 

So are you saying they're charged for the pipe and not for throughput?
In which case why is there such a big negative vibe to this kind of
terminology? eg. http://100best-web-hosting.com/articles54.html and
http://www.findmyhosting.com/truthunlimited.htm?

Again, these sites are refering to bandwidth as the amount of data
transferred over a certain period of time rather than bandwidth in the
sense of how fat the pipe is.