This is an easy one. BandWidth is the amount of data that you can transmit/receive
on your connection to the Internet.

Measured normally in BITS :

1 bit is a ZERO "0" or a ONE "1" and that is one binary digit
1 kilobit ( kb) is 1,000 bits
1 Megabit (Mb) is 1,000,000 bits
1 Gigabit (Gb) is 1,000,000,000 bits

Bytes as in Data Storage or Memory works as follows :
1 byte (B) is 8 bits (b)
1 Kilobyte (KB) is 1,024 bytes
1 Megabyte (MB) is 1,048,576 bytes
1 Gigabyte (GB) is1,073,741,824 bytes
1 Terabyte (TB) is 1,099,511,627,776 bytes

I guess you could measure traffic any way that pleases you Bits or Bytes, as long as you
understand the difference.
a 56kbps modem transfers 56 kilobits per second
(and with data compression even 105 kilobits and more)