To clarify a bit more: bandwidth is the size of the pipe and is generally measured in bps, kbps, or mbps.
Data transfer is the amount of water/date you've sent through the pipe in a month, and is generally measured in kilobytes, megabytes, or gigabytes.
A T-1 line runs at 1.54 mbps, and can generally move approximately 500 Gigagytes of data in a one-month (30 day) period running at full speed.
Jeff