The Gigabyte is the equivalent of 1024 megabytes. The giga prefix originates from a Greek word meaning giant. Often, manufacturers of storage devices use the term to compare the cost per gigabyte of different units sold. Something similar, to what we do in the supermarket, when we are analyzing two similar products served in packages with different volumes. In these cases we compare the cost per pound to find out which one is best. This is a very common term used these days when we refer to disk space or storage drive. One gigabyte lets you store approximately the contents of several books used to fill a shelf about 10 feet long.
The Megabyte is the equivalent of 1024 kilobytes or 1,048,576 bytes. However, the International Measurement System argues that the mega prefix should be used to mean one million. So as not to deceive customers, hard disk manufacturers and other storage devices have begun using this recommendation, and equating the megabyte to 1,000,000 bytes. This allowed to reduce the number of clients, who complained, claiming that their devices had less capacity than the one that was advertised. In the early days of computing, a megabyte was considered a lot of data, nowadays, it is not so. A megabyte lets you store a small book, 100 megabytes allow you to store multiple volumes of one encyclopedia, and 640 megabytes is the space occupied by a music CD.