The Kilobyte is a multiple of a byte unit, the term has historically been used to refer to 1024 bytes (although in some contexts it may also represent 1000 bytes). This difference is related to the fact that the measurement units in computing are based on base 2 multiples (because of the bit). But the term kilo used as a prefix in units of measurement usually means 1000. To address this discrepancy, in 2000, the IEEE (Institute of Electrical and Electronics Engineers) approved new units of measurement, in which the Kibibyte (KiB) would be 1024 bytes, whereas the kilobyte would represent 1000 bytes. However, these new units did not have a great reception by the digital community. A kilobyte represents (approximately) the amount of information needed to store this paragraph you just read.
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.