What is Data Transfer?

Whenever a visitor logs onto an Iternet site, data is sent from web servers to the visitor's computer. This data can be composed of web pages, images, movies, sound files, programs, compressed files, or anything else on your site that can be viewed or downloaded by visitors. When all this requested data is added up, then you have your total data transfer. 
How is Data Transfer measured?
This data is measured in "bytes." So let's first look at what a "byte" is. 

A "byte" is the unit most computers use to represent a character such as a letter, number, or typographic symbol (for example, "y", "6", or "?"). Even characters you do not see such as spaces (" ") and characters comprising HTML tags ("<title>") also are stored in a computer as bytes. 

Here's an example:
For the sake of simplicity let's say you have a one-page web site. Your one web page is 20,000 bytes in size -- or about 20 thousand characters. This web page also contains a picture of your dog. The size of this image is 30,000 bytes. When you add the size of both files together, the total data transferred to view this web page and its image is 50,000 bytes (20,000 + 30,000 bytes = 50,000 bytes). When a visitor comes to your site and views the page, 50,000 bytes of data are requested and then transferred. If your page is viewed 50 times that day, that amounts to 2,500,000 bytes of data transfer. As you can see, these numbers can contain a lot of zeros. That's why we have shortcuts when talking about these large numbers of bytes. Read on to see how this is done. 
Gigabytes, Megabytes, and Kilobytes 
To make it easy to talk about large numbers of bytes, we place kilo, mega, or giga, in front of the word "byte." This makes it easier to read and write these numbers without all the extra zeros. 

What they mean:
kilo = 1,000 
mega = 1,000,000
giga = 1,000,000,000 
 

For example:
10 kilobytes of data is equal to 10,000 bytes. 
10 megabytes of data is equal to 10,0000,000 bytes. 
10 gigabytes is equal to 10,000,000,000 bytes! 
An even shorter shortcut:
10 kilobytes can be written as 10k, 10 megabytes is 10MB, and 10 gigabytes is 10GB. 

Hope this helps!