If it wasn't clear from these responses, it might help to keep in mind that there are two related but different uses of "bandwidth":
One is the size of the "pipe" through which data can flow on a network (so "data transfer speed").
For example, if you have a dialup modem you probably have about a 5KB/s "pipe", and so someone might say you "only have 5K of bandwidth"; if you have a 3Mbit DSL connection, you have about 375KB/s of bandwidth. More accurately, that DSL connection is probably 3Mbit down (to your computer) and 512Mbit up (back out to the internet), so you have "3 megs downstream bandwidth, 512k up".
The other meaning is more commonly used by webhosting companies, and that is cumulative data transfer (generally per month). This is also (and more accurately) called a "quota", or "monthly quota".
This is because webhosts generally serve data as fast as possible, but don't want a popular site to be sucking up all of their network bandwidth (1st meaning) or slowing down their shared servers. So, instead of limiting the stream of data, they limit the total amount of data, figuring that it'll follow an average pattern anyway.
As such, you'll see webhosts offering "300GB of monthly bandwidth", though what they really mean is "300GB of monthly transfer, and some indeterminite but presumably very fast available bandwidth".
Also worth noting: Most ISPs offer fixed bandwidth (1st meaning) and "unlimited" data transfer (2nd meaning). In reality, they often do some sort of checking on the total amount of data you transfer, and if it's VERY high, they may complain. Cox, for example, offers a 4Mbit "unlimited" plan, but "unlimited" actually means 30GB of downloaded data (much less up). Others might actually be unlimited, or just not publish the limit.