Here is the basic of it.

Bit (connection) vs Byte (transfer or storage)
The measurement of the network connections is done in bits as opposed to data transfer and computer storage which we measure in bytes. Eight bits is the equivalent of one byte. Thus a 100Mbps (Mega bit per second) can handle 12Megabytes of data per second.

Units of measurement
K Kilo 1,000 one thousand
M Meg 1,000,000 one million
G Gig 1,000,000,000 one billion
T Terra 1,000,000,000,000 one trillion

To get a meaningful estimate of throughput, divide the data connection pipe by eight.
i.e. A 56K modem can download at a maximum throughput of around 7KiloBytes per second.

Dedicated Pipes (Data Centers)
Pipe Data connection size (Bits per second)
10Mbps 10Mbps
Maximum transfer rates of 1.25MegaBytes of data transfer per second.
100Mbps 100Mbps
Maximum transfer rates of 12.5MegaBytes of data transfer per second.
1Gbps 1Gbps
Maximum transfer rates of 125MegaBytes of data transfer per second.

Bandwidth, How to Choose?
Which is better,
(a) a fixed 2,500 Gigs of transfer per month (a shared 100Mbps pipe, by 20 servers)
(b) an unlimited 10Mbps pipe?

Of course, (a) hands down!
Why?
A 10Mbps pipe can handle a MAXIMUM throughput of 3,240 Gigs of data transfer per month (30 days).

Since almost ALL traffic occurs on some sort of bell curve, your server will start to suffer LONG before it ever reaches that theoretical limit. Think Monash car park during peek times.

At 50% of your theoretical throughput limit, you would have already started suffering long delays/lag time during your peak hours of operations and it is time to move to a bigger pipe.

As a rough idea...
A 10Mbps connection can handle websites of up to 1,600 Gigs of transfer a month.
A 100Mbps connection can handle websites of up to 16,000 Gigs of transfer a month.