Data transfer is the amount of digital data that is moved from one place to another in a given time. Outbound traffic from a web site, with the exception of e-mail, is considered to be data transfer. This is also known as bandwidth, the quota of data transfer that you are allowed each month. Anytime someone downloads, uploads, or visits your website - they are consuming a portion of that quota for the month.
Bandwidth is the amount of data that can be transferred over the network in a fixed amount of time. On the Net, it is usually expressed in bits per second (bps) or in higher units like Mbps (millions of bits per second). 28.8 modem can deliver 28,800 bps, a T1 line is about 1.5 Mbps.
Summarised from Google Definition