There are 1000 megabytes in a gigabyte. This means that 500 megabytes is equivalent to 0.5 gigabyte. Megabytes are used in different forms, such as in memory cards, hard drives, USD drive, Blu-ray disks, DVDs, and even in computer software such as Linux and Mac OS X. Megabytes are also used to define a format of a file such as pictures and music.
Megabytes is commonly written as MB. While other people use Mb, it is not recommended since it can cause confusion with the word megabit. Hence, to avoid this kind of confusion, MB is used to refer to megabyte and Mbit for megabit. This is the basic information about megabytes, but if you wish to know more, a simple google search will show you all the information you need.